Bill Callahan never wants his players to feel too comfortable.

He doesn't care if a guy is a 10-year veteran or a rookie entering his first months in the NFL. He's not going to change the way he coaches, and he's always going to find something to improve when he goes back and watches the tape. To Callahan, the Browns' offensive line coach who's worked in professional football for over 40 years, being comfortable in the sport is one of the quickest ways to derail a career.

So when it comes to coaching the Browns offensive line, a unit that ranked among the best in the league last season, Callahan isn't allowing his players to relax and believe they'll simply coast into 2021 with the same talent and production.

"We've got a long way to go in so many respects," Callahan said Wednesday in a video call with local reporters. "I don't know if you ever 'arrive' or if you're ever the best. We just want to be one of the top caliber lines in the league."

Callahan's job at pushing the O-Line to be even better in 2021 will be a tall task.

That's not because the group doesn't have talent or commitment to be better. It'll just be difficult to significantly top the consistency and cohesion from last season, when both right and left guards Wyatt Teller, Joel Bitonio and RT Jack Conklin, who earned All-Pro honors, were among the highest rated linemen on Pro Football Focus. C JC Tretter and LT Jedrick Wills Jr. both performed well, too, and were reliable pieces in helping pave the way for the Browns' third-best rushing attack in 2020.

So where can the group get better? Well, Wills was only a rookie last season and is expected to make a big leap in Year 2, so the left side of the line could be in better shape. And even though the overall O-Line numbers were stellar last season, the starters played in only eight total games together, so imagine what could happen with a full-strength lineup for most, if not all of a season.

The high results last year despite the injuries are kudos to the depth, which figures to be strong again in 2021.

It's also a tribute to Callahan, who has been constantly praised by head coach Kevin Stefanski.

"He is very sharp. He is meticulous," Stefanski said. "He coaches his guys hard, but he is very fair. He has a great demeanor. He is excellent as a game planner. He has been through a lot and he has seen a lot as a coach, so he provides great value there to myself and to the other assistant coaches. Just on a specific technique level, he and (assistant offensive line coach) Scott Peters do an outstanding job with our players in getting them better and utilizing techniques that get them better. He is obviously outstanding in that regard."

Anyone watching practice knows when Callahan is coaching. He's constantly on his hands and knees to get to a player's level when they're crouched at the line of scrimmage, and his voice can carry the length of a football field when he needs to ensure a message is strictly delivered.

Players know Callahan is always looking out in their best interests. He's been coaching in the NFL since 1995, and there are more than a few reasons why he's stuck around in the professional ranks and has developed some of the best offensive lines over the last decade.

"He gets after us, which at times is frustrating, but it's good," Conklin said. "It's what you want. He expects perfection, and good isn't good enough. It definitely drives us and makes us better. It's not just "get the job done." It's 'hey, you blocked him here, but you could've done this, this and this and it would've made your job easier and you would have looked better."

One of the players who benefited most from Callahan's teachings in 2020 was Teller, who was one of the top breakout players in the entire league and delivered one of the best seasonal performances from any offensive lineman.

Teller attributed much of his success to the work of assistant O-Line coach Scott Peters, a technique guru who spent several hours refining the hand and feet placements of Teller in the 2020 offseason. But Callahan, of course, was right there with him, too, and took every opportunity to correct Teller and mold him into a beast.

"He doesn't let you get complacent," Teller said. "It's impossible. He'll yell at you so much, you won't even know which way is up. He definitely stays on you and he definitely gets you right, and I'm thankful for that because for a lot of the guys in the room, that makes a huge difference. His eyes are so minute on the smallest details and what's going to help you."

The average fan might watch the current Browns O-Line and see nothing major to fix, but not Callahan. His list of things his group needs to improve this season is long — and every player knows the length of the list won't be changing any time soon.

"We spent the entire offseason tweaking what we're doing up front," he said. "We're never pleased in that regard. We're always trying to strive to do things differently than what other people do. I'm not saying it's better, it's just a little bit different. I think that's what you have to do in order to keep your edge."