It has been a while since we last heard from Bill Davis, Eagles defensive coordinator of mystery.

He was hired in early February, answered about 30 minutes of questions from reporters a few days later, and vanished like Jason Babin against the run for three months.

Aside from coach Chip Kelly, it's safe to call Davis the most important coach on the Eagles staff. He'll have free rein over the defense - Kelly has said that he gives his coordinators autonomy. He'll call the plays on game day and ultimately be responsible for setting the tone on that side of the ball.

In other words, the defense, good or bad, will take on the personality of its leader.

"I think it naturally does," Davis said Thursday.

Philadelphia hardly knows Davis, except for perhaps his two failed stints as a coordinator with the 49ers and Cardinals. But those who have worked with him have described him as organized yet open-minded, even-keeled yet fiery.

"I think I'm pretty steady until it's time not to be," Davis said. "Effort's a choice. So when I'm around guys that don't choose effort, I've got no tolerance for that. Those are things that will set me off more than anything, or a team that is turning on itself."

Davis is inheriting a defense - at least the pieces that remain - that was at times guilty of not giving effort and ultimately turned on itself. The 47-year-old is taking over a unit that played one way and is now being asked to play another way. And he's taking over as it is being rebuilt brick by brick.

Needless to say, it might take some time for Davis to get the defense humming.

"This year is absolutely the hardest transition year we're going to have," Davis said. He later added: "The learning curve in the first year, you have to understand it, it's a foreign language that you speak."

The biggest challenge right now has been transitioning from a 4-3 scheme. Kelly and Davis have been evasive about their plans up front. There have been a lot of 3-4 looks at two open practices, but both have cautioned that it was only a small sample.

So how would Davis, who has mostly a 3-4 background but ran what was called a 4-3 "under" in Arizona, describe his front- seven scheme?

"Multiple is the best way," he said. "I know you guys are tired of that answer. I know you want to hear one or the other or something. What we're doing here is we're taking that wide-nine 4-3 and we're moving in the direction of the 3-4, but where we stop is yet to be determined by the players we have."

Kelly has said that he wants a versatile defense. The 3-4 generally allows for more looks because the outside, pass-rushing linebackers stand up. Because both are in a two-point stance, the opposing offense has less idea who the fourth rusher is if the defense is in its base.

Trent Cole and Brandon Graham, former defensive ends, are being asked to become outside linebackers. Because the position usually requires some dropping into coverage, some have wondered how much success they will have in the transition. Davis stressed that players won't be asked to do something they can't do, but the Eagles may have no choice with some prominent square pegs.

"I've seen seven weeks of them standing up and going in a three-point [stance], and they can do either one," Davis said. "At first, it looked unnatural to them because it was, because they haven't done it. Now they have done it for a couple weeks."

There will be other changes, as well. The defensive line will be larger and have more responsibilities in stopping the run. DeMeco Ryans and Mychal Kendricks, the middle and weakside linebackers last season, will share the inside.

The secondary, particularly the safeties, won't have as much to do with helping stop the run as it did in the wide-nine. Nate Allen and Kurt Coleman, last year's starters, often left the corners out to dry because they had to fill certain gaps.

"I think any time that you ask the secondary to be primary B- or A-gap run defenders, you're just asking for trouble on play-action and deep balls," Davis said.

Davis will probably call more blitzes than his predecessors, Juan Castillo and Todd Bowles. The wide-nine was predicated on the front four's generating most of the pressure on the quarterback, but a hybrid front provides all sorts of opportunities to create pressure.

"We'll pressure anybody on the defense," Davis said. "We've got pressures for everybody. We'll bring anybody we need."

An odd-man front also affords Davis, who said he would probably call plays from the coaches' booth, the chance to disguise his blitzes or create the illusion of multiple rushers, as he said former Eagles coordinator Jim Johnson used to do.

"Now he had some great dynamic pressures, and I've studied a lot of them," Davis said. "They were out-of-the-box thinking. But when you really break him down, it was more guys up in the A-gaps with the illusion of pressure than actually more than four rushers."