Washington Redskins offensive line coach Bill Callahan set the tone on the first day of last spring’s workouts: Linemen under his tutelage would stay on the field for extra work after practice daily, doing drills designed to chisel each session’s lessons into muscle memory.

Eight months later, the extended work days have become such a part of the offensive line’s routine that on those rare occasions when Callahan grants a post-practice reprieve, his players stay behind for more drills anyway.

“It has been ingrained in us to get extra work,” veteran Kory Lichtensteiger explained Thursday. “You’d think, going into Year Eight [in the NFL], that you’ve heard pretty much everything, but that’s not necessarily the case. Coach Cal came in and has got specific techniques for every scenario. The way we prepare, both in the classroom and on the field, he really tries to get us to learn everything inside and out.”

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On a Redskins team that has undergone a striking upgrade in maturity, professionalism and performance in one year’s time, the offensive line’s transformation may be the most significant.

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Callahan’s group hasn’t gotten the headlines that have followed quarterback Kirk Cousins’s emergence, nor the acclaim of play-making targets DeSean Jackson and Jordan Reed. But its steadiness and effectiveness — despite youth, injuries and a revolving cast — have emboldened Coach Jay Gruden and offensive coordinator Sean McVay to open up the playbook in the second half of season.

In many respects, the line’s ability to give Cousins time to operate lies at heart of the Redskins’ improbable success in doubling their win total from last season and vaulting from the bottom of the division to NFC East champions.

With Sunday’s regular season finale at Dallas left, Washington’s offensive line has allowed just 26 sacks — seventh best in the league, and down from 58 in 2014.

[Matt Jones misses practice with hip issues]

Cousins’s quick release has played a part. Robert Griffin III, who held on to the ball longer in his nine games last season, accounted for 33 of those 58 sacks.

But left tackle Trent Williams, the line’s anchor and the team’s lone Pro Bowl honoree, says Callahan deserves considerable credit for the unit’s improved play.

“He works us pretty hard, but it’s just not working to be working. It’s not busy work,” Williams said. “All of it plays a part in what we have to do in the grand scheme of things. He relates to the players well. He knows how to fire people up; he knows how to strike that fire in you. His charisma is infectious. Everybody feeds off of it.

The Post's fantasy football experts, Gene Wang and Des Bieler, look ahead to 2016 and project the players who will be taken in the first two rounds in 2016. (Thomas Johnson/The Washington Post)

“We pay a lot of attention to detail, to the small things — especially in the passing game,” Williams continued. “When you have a pocket passer who you know likes to set his feet and deliver the ball, you’ve got to keep him clean or else he won’t be able to do what he’s supposed to do with confidence.

“That’s important for Kirk. And that has been our sole focus: just keeping him clean and giving him a chance to deliver the ball.”

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Callahan, 59, has steered clear of the spotlight this week, declining interviews as the Redskins prepare for Sunday’s game at AT&T Stadium. It will mark his first trip back to his former employer’s home field since he chose not to re-sign with the Cowboys after three seasons as Coach Jason Garrett’s line coach and, in 2014, offensive coordinator.

It’s not a must-have victory, given that the Redskins have secured home-field advantage for the first-round playoff game they’ll host Jan. 9 or 10. But having finally strung together three consecutive victories to reach 8-7 after a 2-4 start, it’s a victory that Gruden, as well as many Redskins assistant coaches, players and fans, would love to get — especially since Dallas upset Washington, 19-16, at FedEx Field just four weeks ago.

Said defensive end Ricky Jean Francois: “It’s the same team that beat us at our stadium on center stage! We lost on Monday night; nobody else on TV but us! We lost at FedEx Field; don’t nobody play on that field but us! Just to lose the game how we lost it — we need to go back out there and play it again!”

Callahan was widely credited with the outstanding offensive line play behind the Cowboys’ 12-4 season in 2014. The unit protected quarterback Tony Romo well and helped DeMarco Murray to 1,845 yards and the NFL’s rushing title.

But this season’s Cowboys are a shadow of that 2014 squad. Murray was allowed to depart, and the running game’s production has dropped from 147.1 yards per game to 119.3. Romo, now out for the season, missed all but four games to a twice-broken collarbone. If Callahan’s departure represented a setback, his addition to the Redskins’ staff has been a boon.

“He’s the guy behind all of what makes [the offensive line] go,” Cousins said. “It builds a culture and a mind-set that to be champions and to play at a high level, we’ve got to do things a little differently, take it a step further. For these guys, it’s not a hobby. They’ve had to really work at it. And I think the results have shown in the way they’ve played. It’s exciting to think where we can continue to go if we just keep working and grinding.”