Bruce Arians has been coaching since he was 22 years old, but the day that defines his career didn't take place until 37 years later.

For Arians, Oct. 1, 2012, changed the trajectory of his life. A longtime NFL assistant coach, Arians had been passed over for dozens of head-coaching interviews while he groomed some of this era's greatest quarterbacks, won Super Bowls and prepared himself to take the reins of a team.

He was 59 years old that day, a Monday, when he was named the Indianapolis Colts' interim head coach while Chuck Pagano took leave to treat his leukemia. For the first time in his professional coaching career, Arians was a head coach, some 24 years after he was fired from his first head-coaching job at Temple University.

"It's bittersweet that it took something like that to get an opportunity," Arians told ESPN.

That day set into motion a whirlwind of a season, often described by those who were part of it as magical, that led to Arians not only getting his first head-coaching interviews, but his first NFL head-coaching job when the Arizona Cardinals hired him on Jan. 17, 2013.

Arians believes, five years later, he wouldn't be the Cardinals' head coach without that day.

"No, it doesn't happen," Arians said. "I'd probably still be coaching Andrew [Luck], hopefully."

That day is still "surreal" to Arians' son, Jake, who played an integral part in Bruce making the transition from offensive coordinator to head coach that week.

"I don't know a strong enough of a word to look back five years later and say, 'Yeah, if that doesn't happen, he's probably still the offensive coordinator in Indy if he wanted to coach that long,'" Jake said. "He loved Andrew that much that he'd be happy doing that.

"It's been a crazy four years [in Arizona] and it all came because of that. I don't know how to describe it. It's just unbelievable."

As Arians' Cardinals take on Pagano's Colts in Week 2 (1 p.m. ET, Fox), let's go inside the 24 hours that changed Arians' life, setting it on a path that those who have coached and played under him believe came years too late.

"He showed he should've been an NFL head coach long before his 60s," former Colts general manager Ryan Grigson said.

Sunday: The night before

Arians was sitting on the back porch at his Georgia home on Sunday, Sept. 30, 2012, taking in the last few moments of the Colts' early bye weekend before he had to return to Indianapolis to prepare his 1-2 Colts against the 2-2 Green Bay Packers, when his phone rang.

It was Pagano.

On the Wednesday before, Pagano popped his head into an offensive meeting to wish the staff a good weekend off. He had a doctor's appointment he had to get to.

At the time, his staff didn't think anything of it. That would all change a few days later.

Grigson was among the first within the organization to know Pagano had been diagnosed with leukemia. Pagano called Grigson and told him to sit down. The news struck him hard. Harder than most. Brain cancer had taken his father when Grigson was a kid, so he knew first-hand the awful effects of the disease. In the 25 years he and his wife, Cynthia, had known each other, Grigson still believes she had never seen him as emotional as he was after he told her about Pagano's diagnosis.

When Grigson called Colts owner Jim Irsay to relay the news, Irsay was driving. Pull over, Grigson said.

Once the shock had worn off, Grigson and Irsay had to decide who would take over while Pagano underwent treatment. As soon as Pagano knew he would have to leave the team, he had a conversation with Grigson and Irsay.

"We got together on it," Pagano said, "and I said, 'This is the guy that needs to be the guy.' There was no hesitation on Mr. Irsay's part or Ryan's part."

"It was really a no-brainer," Grigson said. "He [Arians] was the only person on our staff that had any head-coaching experience at all. We had a lot of youth on our staff. Chuck and I were both a first-time general manager and head coach. Then, you know, B.A. had an obvious presence about him, and that's palpable. The presence he has and the confidence he coaches with and instructs with, he just made it a real seamless, easy choice in that regard to lead us while Chuck was out."

Pagano told Arians the news that evening and then told Arians that Irsay and Grigson wanted him to be the interim head coach.

"I almost fell off the porch," Arians said. "You just start praying. Football takes a backseat. You're just praying for a great friend that's going to be OK."

Arians got on a flight back to Indianapolis that night and began researching anything and everything he could about leukemia. He wanted to know everything about it, and then some: What could Pagano's prognosis be? What would Pagano go through?

The first call Arians made was to his son Jake, a former NFL kicker who was living in Birmingham, Alabama, at the time. Jake knew his father needed someone to trust, to lean on, to support him the next day and the rest of the week. He hung up with his dad, and his wife, Traci, looked at him. "You're going?" she asked. There wasn't even a moment's hesitation. He was out the door by 6 a.m. the next day to make the seven-hour drive up I-65 to Indianapolis.

"There wasn't any talking about it or thinking twice," Jake said. "I had to go be there. Whatever I'm needed for, I'll do."

At some point Sunday night, word spread throughout the Colts' coaching staff that a 7 a.m. staff meeting had been called. The coaches immediately knew something wasn't right.

"Chuck didn't do that," former Colts offensive line coach Harold Goodwin said. "So at that point, I knew something was up."

Goodwin, now the Cardinals' offensive coordinator, called Arians immediately, who told him about Pagano's diagnosis. As the news spread among staff, the players were in the dark about Pagano's health, former Colts quarterback Drew Stanton said.

Arians arrived at his downtown Indianapolis home late Sunday to get one last night's sleep before his world was about to change.

Monday: The morning meetings

The rest of the Colts' staff found out the news at an emotional 7 a.m. meeting, an hour before the team would be informed.

When the 8 a.m. meeting didn't start on time, the players began to wonder what was going on. Then Irsay and his three daughters -- Carlie Irsay-Gordon, Casey Foyt and Kalen Jackson, all vice chairs and co-owners of the Colts -- walked in. So did Grigson and the rest of the front office. Trainers were there. Doctors were there.

"You could tell something important was going on," Stanton said.

"Immediately your antenna kind of goes up," former Colts offensive lineman Joe Reitz said.

"You could feel the aura in the room," former Colts safety Antoine Bethea said. "What was he going to say?"

"He didn't show up to meetings, and it was getting late, and we were like, 'Where's coach at? Where's Coach at?'" Colts receiver T.Y. Hilton said.

Then the talking started. Irsay told the team Pagano was diagnosed with leukemia and Arians would take over in the interim. Then the doctors spoke. They walked the players through everything Pagano would go through over the next few months, the treatments, the procedures, everything.

"I don't think anybody was expecting that," Bethea said.

"It was just kind of a 10-ton weight fell down on everybody," Reitz said. "Everybody's trying to process the information."

"I almost fell off the porch. You just start praying. Football takes a backseat. You're just praying for a great friend that's going to be OK." Bruce Arians after hearing Chuck Pagano's diagnosis

Shock spread throughout the room, Stanton recalled. The game -- both of football in a general sense and the one Sunday against the Packers -- was put into perspective. That was just a game. This was life. This was potentially death.

"You were broken," said James Bettcher, Pagano's former assistant who was also a defensive assistant.

When Irsay spoke, he placed one of the two largest expectations on Arians' shoulders that Arians claims he ever has experienced: The Colts would beat the Packers that weekend and then bring the game ball to Pagano in the hospital.

Talk about pressure. The only other time Arians had felt that much pressure to win a game was heading into Bear Bryant's last game at the 1982 Liberty Bowl. Now he was in charge of making sure his team won.

"We were probably 20-point underdogs," Arians said. The Colts were officially 6.5-point underdogs that weekend. "I said, 'Thanks, I didn't need that, too.'"

After that meeting, Arians took the floor to address the team in a football meeting, of sorts.

Arians isn't a "big speaker," he said. After almost 25 years since the last time he stood in front of a team as a head coach, Arians had not envisioned the moment again -- and especially not in that scenario that morning. He didn't prepare a speech.

He estimates now that he talked for about five minutes. And it's what he said that still has a profound impact on those who heard it. Arians made two pledges that morning: To keep the light on in Pagano's office until he came back to work, and to change nothing.

It was Pagano's team, Arians stressed, and that's how it would stay. He told the team he wasn't the head coach. That was and always would be Pagano. The only difference, Arians said, was that he'd be deciding if the Colts went for it on fourth down.

"Everything stayed the same," Arians said. "Chuck's locker was dressed for the game every Sunday. No one sat in his seat on the bus or anywhere else. I was never the head coach. I didn't want to be the head coach. We had the coach. My leadership role just changed a little bit."

That speech, that approach, that mindset, that philosophy dictated the rest of the season right then and there.

"I just remember him being very calm, which we needed at that time," Reitz said. "I remember him being very confident, which we needed at that time."

"It was one of the best displays of leadership I've ever seen," said Bettcher, now the Cardinals' defensive coordinator. "He was such a rock. When everything was. 'How are we going to do this? What are we going to do? We need to do this,' he was very matter of fact.

"You realized what kind of man B.A. is."

Arians would stick to the process -- Pagano's process -- he told the coaches in a brief meeting right after he addressed the players.

"I told them, 'Do your job. I'm not doing yours. We know what Chuck wants. He set a thing in motion that we're going to follow,'" Arians recalled.

From there, the Colts' typical schedule kicked into gear. Since it was the Monday after a bye week, Indianapolis had scheduled an extra practice. After that morning's meetings, the players broke into their position meetings. Then they had practice and ate lunch.

"It was such a different day," Reitz said. "Such a weird day. We're all texting our wives and family and letting them know and praying for Coach."

"It kind of knocked us off our feet," Bethea said.

By the time Jake arrived from Birmingham early that afternoon, Arians was already in football mode. He was calm, and he was more concerned with Pagano than the game.

But one of the few things Arians was nervous about that day was addressing the media. He didn't want the news conference to be about him. He wanted it to be about Pagano.

Jake was Arians' backbone all week. He sat in his office that first day, listening to everything Bruce wanted to say. Arians could process the day on his own terms. There'd be no judgment from his son.

Jake noticed a different version of his father. Bruce was opening up to Jake more.

"It was really humbling to see somebody in that position," Jake said. "A lot of people would be excited -- not necessarily excited Chuck was sick but excited to have the opportunity to be the guy and that kind of stuff. He didn't want it then. He wanted it to be Chuck's message.

"I think he had the burden a little more of being strong for everybody."

Monday: The first practice

All morning, players tried to process what they had just heard.

Attention spans in position meetings were short. As much as some guys tried to fight it, their minds wandered to the depths of cancer's reach. A distraction finally came when they took the practice field.

"I think in between the lines is where you kind of found some solace just because it's two hours, and you're focusing on playing ball and what play you're running next and going out there and competing," Reitz said.

"Sometimes I think that can be therapeutic."

That practice was the first time he was in charge of what every player on the field was doing since he was fired by Temple in 1988.

Arians tried to keep everything about that practice as normal as possible. He called the plays, just like he did every day before that as the offensive coordinator. He let quarterbacks coach Clyde Christensen handle Luck and the rest of the quarterbacks.

But for as much as he didn't want things to change, they did. He became more diplomatic. He wasn't just an offensive coach anymore. He had to make sure the defense was doing its job, too.

"It was a little different because he went from talking a bunch of trash to the defense the whole time to now being somewhat excited when we turned the ball over," Stanton said. "We were all of a sudden looking like, 'What do you mean?' I just remember him saying, 'Nice, good job. Way to get our hands on some footballs today.' I was like, 'Huh?'"

Gone were the days of Arians wearing black to practice to mourn how poorly the defense played against his offense.

Gone were the days of him saying the offense had a bad practice if they committed too many turnovers.

Gone were the days of him trash-talking the defense.

"We had to simmer down a little bit," Bethea said with a smile.

"He had to flip the switch and be impartial," Stanton added. "You could tell it was a little bit difficult for him at first. I still don't think he likes doing it because he was an offensive guy at heart."

It comes with the territory of being a head coach.

Reitz remembered how hard Arians coached that first practice. There was more of a purpose that day. It helped the players keep their mind on something other than Pagano's illness.

"He's just an instinctive, natural playcaller that's got a lot of guts and that kind of swagger he helped instill in our team when we really didn't have a right to have any at that time, but we were playing for a cause and that goes a long way in sports, I think." Former Colts general manager Ryan Grigson on Arians

"We were going to focus on ball," Reitz said. "We're going to focus on every little detail and that's going to give us a little bit of a reprieve."

Arians made another minor change at the end of that two-hour practice.

Instead of having the customary four or five players break down the team with phrases such as "hard work" or "win," Arians told the team they'd break down practice with a new chant: "1, 2, 3, Chuck."

"That," Arians said, "was our rally cry from then on."

Monday: The night calm

As that first afternoon turned to night, the Colts' facility in Northwest Indianapolis had emptied.

Bruce sat at his desk in his office, scripting plays for Sunday's game, with Jake sitting nearby. Relief "that [Bruce] had gotten through the day" had settled over Jake by that point.

As Bruce sat there, one constant thought went through his mind: "How the hell are we going to beat the Packers?" He didn't ask for that type of pressure, but once Irsay said during the team meeting that the Colts would bring Pagano the game ball after beating Green Bay later in the week, Bruce knew he had no choice but to figure out a way to win.

Bruce turned to Jake and said he had an idea on how to beat the Packers: Go no-huddle.

Jake looked at his father like he was nuts.

"He goes, 'You got a rookie,'" Bruce said with a chuckle. "I said, 'It's our only chance offensively to score with them.'"

Jake responded: "'I know you love this kid and you think he's the greatest thing you've ever been around, but you're talking about playing Aaron Rodgers and going no-huddle.' It was his fourth game. And he goes, 'Yeah, I think it's the only way we can beat them.'"

Jake wasn't about to disagree with one of the most respected offensive minds in the NFL, even if it was his father.

"He sold me on it in about 30 seconds," Jake said. "I was like, 'You know better than anybody. If you have the confidence in it, they're going to follow you. You have to run with it."

The game plan was set.

But Bruce wasn't done worrying. It got to the point where the stress was weighing on him.

While he was scripting, Bruce felt something was wrong.

"S---, I'm about to have, I thought, a heart attack," Bruce said.

He called for a trainer, who examined Bruce. He was fine.

"It really wasn't anything other than panic," Bruce said. "I said, 'We could do this. We'll get through this.'"

Each day got easier and easier, Bruce said.

"He felt that pressure," Jake said. "The biggest thing I remember from that day was just: You made it through the press conference. You made it through addressing the team. You made it through practice. You got the game plan in. It's time to go get a little sleep and go do it again tomorrow.

"You checked all those boxes for the time you were worried about, and it's going to be easier tomorrow. That's probably the biggest thing I remember. That little bit of a deep breath of: We got through this one. None of them are going to be that hard again."

Bruce and Jake left the facility around 10:30 p.m. and headed back downtown to Bruce's place.

They had to do it all again in the morning.

Epilogue

The entire week -- the emotion, the stress, the pressure -- all built up to Sunday afternoon inside Lucas Oil Stadium.

With Pagano in the hospital, Arians walked the sideline that day as a first-time NFL head coach, maybe not in title but at least in role. Despite the calm, collected face he had been showing all week, nobody knew what kind of head coach Bruce would be. He'd never been given the chance to show what he was capable of since he first became an NFL assistant in 1989 with the Kansas City Chiefs. Countless jobs passed him by.

Grigson could see what kind of head coach Arians would be before anyone else did.

"I just think his overall instincts ... he's a tremendous playcaller that's an instinctive playcaller," Grigson said. "He's not a guy that has to go through a flow chart on third-and-2 on what to do. He's a guy that I think can draw it up in the dirt. He's just an instinctive, natural playcaller that's got a lot of guts and that kind of swagger he helped instill in our team when we really didn't have a right to have any at that time, but we were playing for a cause and that goes a long way in sports, I think."

Arians, however, was still learning how to be a head coach. When he was at Temple, he was averse to delegating. The work load of doing everything led to him suffering from migraines painful enough to hospitalize him.

When he was named the Colts' interim coach, Arians knew he had to delegate, Goodwin said.

Arians walked into the locker room for the pregame speech and, like he did on that Monday during the team meeting, kept it short.

"It was like, 'Hey guys, we know we're good. Let's go play. Let's go win this damn game,'" Reitz recalled. "I think just that brevity had an impact on the players."

The first half went about as badly as it could. It ended with Indianapolis trailing 21-3.

Jake was alongside his dad on the Colts' sideline, the first time he had been on an NFL sideline since he kicked for the Buffalo Bills in 2001. He headed to the locker room a couple of minutes before halftime to see if he could get a read on the players and to watch how Bruce handled it.

Jake had always thought he knew what kind of head coach his father would be, but when Bruce walked into the locker room, 30 minutes away from failing Irsay, Jake saw a "different guy that I had never really seen." Instead of entering the room with a head full of steam, ready to yell and scream, Bruce was calm. Eerily calm, Jake said.

Yet he was confident.

Bruce calmly described to the players what would happen at the start of the second half: The Colts would send the opening kickoff into the end zone for a touchback, they would get an interception and then score a touchdown. Then they'd be right back in the game, he said.

Pat McAfee kicked the ball nearly out of the end zone. Three plays later, former Colts cornerback Jerraud Powers intercepted Rodgers. The Colts needed just five plays to go 39 yards and score a touchdown.

It went exactly as Bruce had predicted. The players just looked at Bruce, Jake remembered.

"That belief started," Jake said.

"The biggest play that I think happened that whole season was Jerraud Powers' interception," Goodwin said. "That changed the whole mentality because up until then they were kicking our ass.

"The whole season changed off of that."

The Colts were down 21-10, and added a field goal and another touchdown before the third quarter was over. Just 15 minutes before, the Colts were getting blown out. Now they were down 21-19. They took the lead in a field goal about midway through the fourth quarter. But Rodgers wasn't done. The Packers reclaimed the lead 27-22 with 4:34 to play. Then Luck orchestrated a 13-play, 80-yard drive that he capped with a 4-yard touchdown pass to Reggie Wayne with 39 seconds to go. The second Wayne reached the ball over the goal line, Bruce grabbed Jake and the players mobbed them both.

"I don't cry very often," Bruce said. "I was sobbing on the field just to know that he's getting his game ball."

The Colts didn't stop winning after that game. A 1-2 start turned into an 11-5 season and a berth in the playoffs.

Everything fell into place that 2012 season, in large part because of Arians. He stayed the course. He kept Pagano's process the Colts' process.

And through it all, Arians proved every owner and general manager who decided to never interview him wrong. He went 9-3 as the Colts' interim coach and was named The Associated Press coach of the year, the first time in NFL history that an interim coach won the award. He finally caught the attention of teams who were in the market for head coaches. He was interviewed by the Chicago Bears and the Cardinals, who, 25 years after his last head-coaching job, gave Bruce his first NFL head-coaching job at age 60.

It was the reward for taking an unenviable and unfortunate situation and making the best of it.

"He stayed within himself," Reitz said. "I think he was true to himself and he was true to Bruce Arians.

"He was the perfect coach to take over for that situation, and the perfect coach for us and the Colts to take over while Chuck was gone."