Familiarity is the running theme of the 2020 NFL offseason in Western New York, where the Buffalo Bills will return 18 starters this coming season.

But with the exception of wide receiver Stefon Diggs, even the Bills' biggest additions are already familiar to their 2020 coaching staff.

Cornerback Josh Norman, linebacker A.J. Klein and defensive linemen Mario Addison, Vernon Butler and Bryan Cox Jr. all played for the Carolina Panthers earlier in their careers, adding to the growing list of former Panthers to sign with the Bills. Those players all know general manager Brandon Beane and played under coach Sean McDermott, but it was another former Panther who played a pivotal role in convincing the linemen to come north -- new defensive line coach Eric Washington.

He held the same role in Carolina from 2011 through 2017 before taking over as Carolina's defensive coordinator for the past two seasons. When the Bills hired him this offseason, his former players quickly followed.

"I love coach Washington, mainly because he gave me an opportunity when I got to Carolina. He believed in me," Addison said. "He developed me into the pass-rusher that I am now. ... Coach Eric Washington unlocked everything for me. And he kept pushing me until, bam -- one year I had double-digit sacks.

"He told me if you don't grow in this league and you don't produce in this league, then you're bound to die in this league. Not only that, he and coach McDermott are guys where it ain't all about football. When they're teaching life lessons, it's about real life things on and off the field -- trying to be a better man, better husband, better father."

Addison doubled his career sack total in his first full year under Washington (2013) before beginning a solid six-year run from 2014 to 2019, during which he posted the 13th most sacks in the NFL.

Washington, entering his 13th year as an NFL coach, also had an impact on Butler. The defensive tackle turned in a career year in 2019, posting six sacks in the final year of his rookie contract. Although the former first-round pick's career numbers had been underwhelming to that point, Washington said Butler offers the exact versatility the Bills seek.

"We want to attack the line of scrimmage and Vernon can definitely do that," Washington said. "I think Vernon has matured as an individual and there was strong evidence under some tough circumstances last season with the production he was able to attain. I think he's figured out what he needs to do to win in a system like this, where the front is featured."

Washington said he was not only "excited" for the opportunity to coach Addison and his former players once again, but also to reunite with Beane, McDermott, assistant GM Joe Schoen and director of player personnel Dan Morgan.

That familiarity was a driving factor in his decision to accept the Bills' offer -- a relationship McDermott is grateful to rekindle.

"Eric and I first were introduced to each other when he came to Carolina. I was the coordinator, he became our defensive line coach," McDermott said. "Both of us were rather young in our respective roles in terms of experience, and so I know it's been just really cool to watch Eric grow over the years, and to see him with his family. And then for me to grow over the years in hopefully some similar capacities.

"And now to be reunited in Buffalo is really ... I appreciate it, because he did not have to. He had choices and did not have to come here and he wanted to come here and move his family here, so I think that I'm very appreciative of that and I can't wait to see what value he adds to our defensive line."

Although he has yet to officially move to Buffalo, Washington has interacted with his fellow coaches during the team's offseason. Even their limited time has left him reassured with his new position.

Especially when it comes to working with Bills defensive coordinator Leslie Frazier, who has coached a top-three defensive unit in each of the past two seasons.

"Les Frazier -- his reputation precedes him. I can't tell you how excited I am to be working on a staff that he's leading," Washington said. "So far, first impression-wise, it's just a bunch of guys who are hard-working guys investing in what we're trying to do in terms of helping these players. There were just a lot of things that made me and my family attracted to joining this organization."