Bradley Roby rested on his heels and placed his hands lightly on his knees as his eyes bounced back and forth at the quarterback and the receiver standing a few feet in front of him. His mind quickly ran through his usual presnap checklist — the offense’s formation, the receiver’s split, the personnel groupings — to process the variables.

A second later, Roby was on the go, trailing San Diego wide receiver Tyrell Williams as he darted in front of the Broncos’ linebackers as a pass from quarterback Philip Rivers sailed his way. The ball grazed the fingertips of Williams before falling into the arms of Roby, who dashed toward the end zone for a touchdown.

The mental processor in his head resorted to simplicity: “Score. ASAP.”

“I just expect those plays,” Roby said. “I anticipate them. I kind of visualize them. When they happen in real life, I’m just ready for it. I really think that’s what it is.”

Roby didn’t have to think much en route to his score last Sunday because he already spent more than two years doing so, poring over film, reading and reacting alongside Pro Bowlers Chris Harris and Aqib Talib, and learning with veteran Kayvon Webster and second-year pro Lorenzo Doss.

Denver’s cornerbacks are arguably the deepest contingent in the NFL, and Roby offered a reminder against the Chargers when he made a start in place of the injured Talib. Roby was named the AFC’s defensive player of the week for his performance, while Doss preserved the victory with his fourth-down deflection of a Rivers pass in the waning minutes.

Denver’s depth at cornerback, however, will be severely tested again Sunday night, when the Broncos (6-2) face the Raiders (6-2) in Oakland without Talib (back) and Webster (hamstring). The challenge will be great — perhaps the greatest of the season — with the AFC West lead on the line and the Raiders boasting a pair of receivers, Amari Cooper and Michael Crabtree, who are on track to top 1,000 yards this season.

But if any group is equipped to handle it, the Broncos are arguably the best-suited — for reasons that go beyond the stat sheet or highlight reel.

“We have the best secondary in the National Football League, not because of the starting cornerbacks, but because of the depth that we have,” said all-pro outside linebacker Von Miller. “We got Bradley Roby, Lorenzo Doss, Taurean (Nixon), Kayvon Webster — all these guys that can come in and contribute. … It’s still the No Fly Zone.”

Preparation is key

The routine is tailored but often learned by others and circumstance — by example and the pressure to perform. Harris, an undrafted free agent out of Kansas who wasn’t even invited to the NFL scouting combine, needs little extra motivation to spend many of his waking hours watching game tape.

“I try to know all the first and second downs and screens before I get here (for practice),” he said. “So then, when they tell me what routes and stuff are coming, it’s kind of like review.”

Depending on the opponent, Harris may go back years to study film. Sometimes he takes notes, but mostly he watches and absorbs, storing the information until a quarterback and receiver are staring him dead in the eye on game day. It’s a weekly process that evolved with the help of former safety Brian Dawkins and cornerback Champ Bailey, and has expanded with the arrival of defensive backs coach Joe Woods.

“We used to have an hour of film study together, with just corners and safeties when I first came into the league,” Harris said. “(Dawkins) showed me formations and things like that. Joe Woods sometimes goes back four years, five years to watch tape. Probably even longer. We do our part in studying, but he laps us.”

For Talib, learning to value and examine film required some coaxing from another great, Ronde Barber, his teammate with Tampa Bay.

“I had this little meeting room that I claimed for my own in Tampa and I’d be in there all the time watching film,” said Barber, an 11-year veteran when Talib was drafted by the Buccaneers in the first round in 2008. “He’d come in, shut the door and we’d just sit there and break down film, and I’d tell him a little about what I was looking at, what the quarterback was telling me, what the split of the receiver was indicating and just get a feel for what the offense was trying to do to the defense. I hope that rubbed off. Watching film is a personal thing.”

For Harris and Talib, as well as the rest of the Broncos’ secondary, their film study is a badge of honor. It’s a requisite but also a science.

“Just watching Chris and Aqib and seeing how much they knew about the game — I asked them, ‘How do you guys know all this stuff?’ The film, the film, the film,” Roby said. “I didn’t watch film at all. We watched film in college, but I didn’t know what I was watching. It’s different when you’re looking for certain things.”

The versatility

The depth in the Broncos’ defensive backfield was provided by general manager John Elway, built over the years in the draft and free agency. But the details have been honed in the meeting room.

“We always prepare,” Doss said. “No matter if you’re inactive, no matter if you’re on practice squad, you always stay ready. We expect that out of each other.”

If the athleticism of the Broncos’ cornerbacks is their foundation, their film study and practice is their framework, providing a clear plan of what to anticipate on game days while allowing enough room for change.

Where each player has his own strengths, all are treated the same to offer an array of options — especially when one is out. Talib typically plays outside, using the sideline as an extra defender while chasing the receiver in front of him. The physical toll can be daunting, and the mistakes easily exposed. Roby’s forte is “man” coverage but he has the ability to play outside, in the slot and at safety, if needed.

But few, if any, in the game embody the versatility of the position better than Harris because of his hybrid role as both a slot and outside cornerback. As a do-it-all defender, Harris is tasked with mastering receivers’ routes, understanding the role of his linebackers and other defensive backs, and quickly processing and reacting in space.

“When anybody asks me, ‘Who is the guy doing it kind of like you did?’ it’s Chris Harris,” said Barber, an unofficial architect of the slot corner position. “He’s active on the inside but he’s productive on the outside, as well. It’s turned into a specialized role, that nickel back spot, but he’s the one guy that’s doing both. And it’s hard to do, especially in today’s NFL. It’s probably harder now than when I was playing. But he’s athletic, he anticipates well.”

Having two corners the caliber of Harris and Talib is a luxury few teams have enjoyed.

“They both can play at the line of scrimmage, both can play off coverage,” Barber said. “If I was the coordinator and had two guys who could do all that, it makes it easy to call defenses.”

Woods, who worked with Barber at Tampa Bay before moving to coach cornerback Antoine Winfield in Minnesota and safety Charles Woodson in Oakland, is quick to agree with Barber’s assessment. But his cornerbacks in Denver have a different dynamic and depth that, in his mind, are unmatched.

“Not a knock on the guys I’ve coached in the past because I’ve coached some great players,” Woods said, “but collectively as a group, we can basically do anything.”

BY THE NUMBERS

A snapshot of the Broncos’ recent play at cornerback:

5: Touchdown passes allowed by Chris Harris in the last 50 games, playoffs included.

2: Catches (34 yards) out of nine targets allowed by Bradley Roby against the Chargers last Sunday.

3.2: Philip Rivers’ passer rating when throwing at Roby last Sunday.

58.3: Percent of defense played by Roby this season (331 snaps).

2: Catches (29 yards) out of seven targets allowed by Lorenzo Doss for 29 yards against the Chargers. He also had two pass breakups.

37.0: QB passer rating when throwing at Aqib Talib.

0: Touchdowns allowed by Talib in coverage this season. He allowed only three last season, including the playoffs.

Sources: Pro Football Focus, NFL

CAUGHT IN THE NO FLY ZONE

Sunday night, the Broncos face Oakland’s Amari Cooper and Michael Crabtree, two premier wide receivers. Here is how some of the NFL’s other top-tier wide receivers have fared against the Broncos this season:

Player, Team Rec./Targets Yds TDs

T.Y. Hilton, Colts 4/11 41 0

A.J. Green, Bengals 8/11 77 0

Mike Evans, Bucs 5/11 59 0

Julio Jones, Falcons 2/6 29 0

DeAndre Hopkins, Texans: 5/12 36 0