The big change Dennis Allen brings to the New Orleans Saints defense lies in the tiniest of details.

Allen is a stickler for the details.

From the moment he arrived and took over the Saints secondary this offseason, Allen started putting his imprint on the defensive backfield, a remade unit trying to blend elements of the old, the new and the combination of both old and new that came in the form of veteran free agents.

Allen began the task of rebuilding the secondary by teaching his charges to master every little aspect of the position, building all of those little adjustments into a bigger picture.

“He’s big on the details,” rookie cornerback Damian Swann said. “He wants us to know everything down to the smallest part. He wants to make sure you get it, you know it before you go out and play on Sunday. And that’s what it’s about. On this level, it’s not about being the fastest guy or the most athletic guy — it’s about the details.”

Allen’s mark on the defense can be hard to see. The group he takes over ranks at the bottom of the NFL in at least a part of every major category, fresh off a three-game stretch that cost former coordinator Rob Ryan his job and left the Saints looking up at the rest of the NFC South as they hit the home stretch.

But there have been signs of Allen’s influence. Kenny Vaccaro looks like the impact player he was in 2013. Swann and Delvin Breaux, two players who entered the season with little expected of them, have shown promise in their first NFL seasons. Kyle Wilson, left almost for dead by the rest of the NFL, has played solid football.

Allen also has been responsible for putting together the Saints’ third-down packages, and New Orleans ranks sixth in the NFL on third down, allowing opponents to convert 34.78 percent of their chances.

“D.A. is a veteran and sharp coach, and I think a real bright guy,” head coach Sean Payton said. “He’s done a very good job on the nickel and third-down packages — much better than we were a year ago. Last year was awful on that down and distance. ... Now I think along those lines he’s going to take the same approach when it comes to the defense in its entirety.”

Allen has always had an innate understanding of the defensive side of the ball.

Born and raised in Texas, Allen played his high school ball at L.D. Bell High in Hurst, then landed a scholarship to Texas A&M. The Aggies defensive line coach at the time was Bill Johnson.

“He came in, bided his time and after two or three years he became a starter on a very good defense,” Johnson said. “He was a free safety, and he was sort of the quarterback of a very, very good defense in those days — the Wrecking Crew days at Texas A&M. You could tell he loved the game of football.”

Allen briefly spent some time with the Buffalo Bills, then opened his coaching career as a graduate assistant at Texas A&M.

Fast-forward a decade and a meteoric rise that included a stop at Tulsa before taking a job with the Atlanta Falcons, and Allen was on Payton’s list of potential assistant coaches despite the fact that he was still under contract on Jim Mora Jr.’s staff in Atlanta. Payton knew Allen was a secondary coach, but he had an assistant defensive line spot open, and the Saints were competing against Jon Gruden in Tampa Bay, an opportunity that would allow Allen to work with legendary coordinator Monte Kiffin.

“I knew he wanted to take a peek at the Tampa thing,” Payton said. “I also knew that there was a good chance that Jim Mora Jr. might give us permission, may not give it to Tampa Bay. I wasn’t sure of that. Sure enough, a couple of days later, Dennis called back, took the position with us.”

Allen spent two seasons coaching the defensive line before moving to the secondary with the Saints.

But the experience proved to be valuable. Coaching the defensive line gave Allen a unique perspective on the defense, turning him into an ideal defensive coordinator prospect.

“He’s very organized, and he’s very disciplined, and he understands football,” Johnson said. “He doesn’t just understand one part of it; he understands the overall working of a defense, from the front to the midsection to the back end of it, and he has a good feel for it.”

Allen spent only one season as a defensive coordinator in Denver before getting hired as Oakland’s head coach.

But that time was plenty to build Allen’s reputation as a coordinator. When the Saints hired him in the offseason to coach the secondary and take a look at the defense, the widespread speculation was that New Orleans hired him to effectively usurp the position from Ryan.

“It wasn’t anything that was thought about or discussed,” Allen said. “Obviously, Sean wanted me to come in here, and I wanted to come back and be a part of it. I wanted to try to help get the defense better. All of us, players, coaches, everybody — we all have our autograph on where we are at right now as a football team, and specifically as a defense. We have gotten to work to try to change that image and that picture.”

What Allen can do to change a defense mid-stream remains to be seen. New Orleans made the move over its bye week, handing Allen an extra week to prepare for Sunday’s game at the Houston Texans (noon, Fox). But that’s not enough time to make wholesale changes.

“We are going into Week 12 of the season, so it is not like you can just go in and just wholeheartedly change everything that you are doing schematically,” Allen said. “We have certain principles that we believe in, and those core values and core principles won’t change. There may be a few things that might be a little bit different. Obviously, my personality and how I call a game is probably different than Rob’s personality was calling a game or anybody’s personality calling a game.”

Expect Allen’s personality to be a little different than Ryan’s. Ryan was always so colorful in interviews; Allen isn’t a walking soundbite. On the practice field, though, Allen can have boundless energy, the players said, always on the move.

And then there’s the details.

Ryan’s defenses were often marked by mistakes over the past two seasons: substitution issues, problems getting the call into the entire defense in time and problems with misalignments and missed coverages. Allen, according to the defensive backs he has been coaching this season, spends most of his time trying to eliminate uncertainty by going over the details with a fine-toothed comb.

“All of it,” Swann said. “Whether it’s the fundamentals, whether it’s run fits, whether it’s leverage, whether it’s technique — it’s all those things. Everything is so detail-oriented that there’s no reason for you not to know what to do.”

Now it’s time to see how much of an effect Allen’s personality can have on short notice.