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Ronald Darby has nothing to worry about when he steps onto a football field. But that doesn't keep the pregame butterflies away.

"I'm a little nervous before each game," he admitted.

Make no mistake: Darby is never intimidated, even when facing Odell Beckham Jr. or Brandon Marshall. But he has always dealt with locker room jitters. Even in college, when his Florida State Seminoles rarely had much to worry about. "It's like I can't even help it," he said. "Even when we were playing a D-1AA school, I was still nervous."

"It's probably because of the fact that there are thousands of people watching," Darby explained. "I always get that nervous feeling."

Darby had every reason to be nervous as a rookie starting cornerback last year in Buffalo. Teams challenged him from the moment he stepped on the field in the preseason. He responded to those challenges with 21 pass breakups, two interceptions, All-Rookie notice and a spot on the league's All-Under-25 team.

So he wasn't exactly playing scared. Even though he did not know his position well. And he slammed into the rookie wall late in the season.

Darby is healthier and better prepared this year.

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Opposing wide receivers are the ones who should be nervous.

Seeing the Big Picture

The Bills defense didn't look good on the practice field. On the injury report, it looked even worse.

It was early August, and while the Bills offense kept getting good news (Sammy Watkins is back! Tyrod Taylor signed an extension!), the defense couldn't catch a break. First-round pick Shaq Lawson underwent shoulder surgery in May and would not return for weeks. Second-round linebacker Reggie Ragland tore his ACL. The team was scrambling to find linebackers and edge-rushers.

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Hope for the Bills defense rested with the healthy young secondary, primarily the cornerback tandem of Darby and 25-year-old Stephon Gilmore. Secondary coach Tim McDonald smiled broadly when asked about the duo. "I truly expect Ronald and Steph Gilmore to be two of the best in the league," he said.

Gilmore agreed with the assessment. "I think our group is at the top," he said. "I think we as a group don't get a lot of credit, but I think we play at a high level."

So it was discouraging to see both cornerbacks struggle in training camp drills. Gilmore gave up a handful of easy touchdowns during my August visit. Earlier in camp, Darby had some noticeably rough outings when matched up against bigger Bills receivers like journeyman Greg Little.

Training camp observations can be highly misleading, however. Head coach Rex Ryan explained that he creates mismatches and bad situations on purpose during training camp. Darby, for instance, was getting an advanced course on covering Dez Bryant-sized receivers in the red zone when he gave up the touchdowns to Little.

"Those are those big bodies, and that's what you've got to learn to stop on defense," Ryan explained. "I know the competitor that [Darby] is," Ryan added, "so he probably can't wait to get to practice tomorrow."

Ryan also drilled his base defense against all sorts of offensive personnel packages—a 4-3 defense against a spread four-receiver formation, for example—to prepare Gilmore and Darby for situations where they might not get safety support.

Darby proved to be an excellent man-to-man cornerback as a rookie last year. He allowed his receivers to catch just 54.3 percent of passes thrown to them, according to Pro Football Focus. Darby, Gilmore and the Bills secondary frustrated stars like Beckham (five catches, 38 yards, zero touchdowns) and Marshall (3-23-1 in Week 10).

But there is more to playing cornerback than just locking down a receiver in man coverage, which is why Bills coaches are teaching their cornerbacks to adapt to unusual circumstances.

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"He's understanding the big picture," McDonald said of Darby. "It's not all about man-to-man. Every now and then we've gotta play some zone, and you have to be able to see more than just your man.

"Steph is pretty good at it. Darby's now getting it. That's what he's been working on."

Darby is eager to learn. "A lot of times last year I would just line up and play," he said. "I would just play one-on-one. But knowing where your help is coming from and all of that can help you make more plays and be a better player."

Darby said he spent the offseason working on recognizing formations and tendencies. He worked on pre-snap communication during minicamps.

The trick is to not let the cerebral part of playing cornerback overwhelm the instinctual part. "We want to do it in increments," McDonald said. "Baby steps. We don't want to go too far away from what made him special last year."

That's a wise policy, because Darby was pretty darn special last year.

Turning Heads

Darby was ready for NFL-caliber competition when the Bills drafted him in 2015. At Florida State, he played in a secondary that also featured Jalen Ramsey, LaMarcus Joyner, Terrance Brooks and P.J. Williams, all of whom have been drafted in the first three rounds over the last three years. During any given practice, Darby was likely to cover Kelvin Benjamin or Rashad Greene, with Jameis Winston trying to get them the football.

"Practice was harder than the games," Darby laughed. "We pushed each other through practice, battling to make plays. It prepares you for the NFL. You get used to competing and going against top competition."

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Darby, the 50th player (and seventh cornerback, and fifth Seminole) selected in the 2015 draft, climbed to the top of the Bills depth chart almost immediately. He started in his first preseason game, allowing a touchdown to a familiar foe: Benjamin, who outleapt his fellow Florida State alum in the corner of the end zone.

Darby rebounded from his debut with two interceptions of Josh McCown in the first half of a preseason matchup against the Browns. That was followed by a debacle against the Steelers in which Darby gave up two long receptions and a 35-yard penalty while chasing Martavis Bryant all over the field.

Darby remembers each of those rookie learning experiences, good and bad. "I just kept practicing. I knew where I was making mistakes. I knew going into Week 1 that I wanted to be completely ready."

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Ryan didn't want Darby riding an emotional roller coaster between big games and bad ones. "He gave me help when I needed help," Darby said of a head coach better known for bombast than gentle guidance. "I didn't come out doing well right away. But he was patient with me and told me what I needed to work on."

Specifically, Darby worked on turning his head in coverage so he could play the ball in the air. "I was trying to get into perfect position, and I didn't want to turn my head around. It made me give up some plays that I could have possibly made."

It's a problem that plagues many rookie cornerbacks. College defenders can jostle receivers in downfield coverage without drawing flags, so many of the best defenders develop a habit of playing the receiver instead of the ball. It's also an easy problem for a defender as talented as Darby to correct.

"I could see if you were five yards behind the receiver, it would be a problem," Darby said of learning to turn and locate the football in flight. "But if you are up in good position on the receiver, and your biggest problem is not turning around, all you have to do is train someone to do that. Then you've got your coverage skills down."

Something else changed for Darby before the start of his rookie season. "The tempo changes from the preseason to the regular season. I didn't feel the same adrenaline in the preseason as I felt in the regular season. It's a different atmosphere."

Maybe those nervous butterflies are a good thing.

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Darby intercepted a pass from Andrew Luck to T.Y. Hilton in his first official NFL start. He added another interception two weeks later against the Dolphins. By the time he faced the Giants in Week 4, Darby was ready to contain Beckham, limiting the superstar to a handful of short catches. "When you're going up against the best, it brings the best out of you," he said. "[Beckham's] one of the best players in the league, so you have to come prepared."

Darby's role in the defense became more important as he went from Gilmore's protege to Gilmore's peer. "At first they were putting Stephon on the No. 1 receiver and me on the No. 2," Darby said. "As the season went on, there wasn't as much rotating. That's when we started making it left and right corner. They trusted me on a side as much as they trusted Stephon."

Darby was on pace to be a Defensive Rookie of the Year candidate. Then he crashed into the dreaded rookie wall.

Wear and Tear

Darby began to feel it around Week 10 or Week 11. He wasn't quite himself. "I realized I didn't have that same twitch," he said. "I remember on game days, I didn't feel like I was recovered. I just felt a little beat up."

It's another common rookie problem: the physical exhaustion that comes from the transition from college football to the NFL. Darby went straight from the 2014 college football season—Florida State played in the College Football Playoff, so Darby trained straight through early January—and into the predraft process of combine and pro-day workouts without a break. From there, it was straight to Bills minicamps, OTAs and training camps. By November, Darby was simply out of gas.

The difference was visible on the field. In Week 10 against the Jets, he allowed just one reception for a loss of two, with two pass breakups, according to Pro Football Focus. But by Week 12 against the Chiefs, he allowed touchdown passes to Jeremy Maclin and Travis Kelce. The following week, DeAndre Hopkins beat Darby repeatedly, and Darby allowed another pair of touchdowns. There's no shame in getting beaten by the likes of Maclin and Hopkins as a rookie, but Darby had been corralling similar receivers just a few weeks earlier.

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Darby missed the Bills' Week 16 game against the Cowboys and had to leave the season finale against the Jets early because of an abductor injury. "Overuse," he explained. "I was probably straining." The Bills won both games, but Darby's absence for most of Week 17 was noticeable. With Gilmore also injured, Marshall and Eric Decker combined for 13 catches, 176 yards and two touchdowns.

The late-season swoon may have kept Darby out of the Defensive Rookie of the Year running. Fellow cornerback Marcus Peters won the award, though Pro Football Focus selected Darby for its version of the honor. Darby may have been better than Peters during his midseason peak, but Peters had eight interceptions and two touchdowns, and he was playing for the playoff-bound Chiefs, while Darby was struggling to stay on the field.

The overexertion and injuries forced the ultra-competitive Darby to do something counterintuitive this offseason. He had to rest. "I gained a little weight," he admitted. "That's from not doing as much running during the offseason, but that normally comes off me. I just wanted to give my body a rest from the load I put on."

That's not to say that Darby goofed off all spring and summer. He said that he focused on flexibility drills, position drills and track work. He stayed away from heavy-duty weightlifting and other high-stress exercises. "Less wear and tear," he explained. He also enlisted the help of the Bills' new assistant defensive backs coach, Ed Reed, who knows a thing or two about proper conditioning.

Whatever weight Darby gained while resting had clearly melted away by training camp. By August, Darby was fitter, smarter about taking care of his body, playing the ball on every snap and learning the finer points of NFL defense. The next step is into the NFL's upper echelon of elite defenders.

The Complete Corner

Darby slid into the second round of the 2015 draft for a variety of reasons. His inability to turn for the ball made him look like an unfinished project. It was easy to lose him in the crowd of great Florida State defenders. His name kept popping up on the periphery of the program's biggest scandals—he was named as a witness in Winston's sexual assault allegations—prompting some character worries.

"I had a chip on my shoulder about all those people drafted ahead of me, etc. That's what helped me work hard and compete harder."

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Darby's story is typical in many ways: the rookie defender with the chip on his shoulder, the young cornerback getting schooled in the preseason, a season of highs and a few lows, an offseason of learning and reconditioning.

For the typical story to have a happy ending, Darby must take another step forward in 2016. Some of that improvement will show up on the stat sheet. "Of course, I want to have more interceptions," he said. "I want to give up no touchdowns and have more breakups."

But much of that improvement will be invisible to untrained eyes, like the knowledge that comes from an offseason of advanced cornerback studies. "I want to understand the game a little bit more. There's things where I felt I could be stronger. I want to become the complete corner, the next big-time corner."

It won't hurt if the rest of the Bills defense improves a little. The Bills recorded just 21 sacks last year, the second-lowest total in the NFL. As good as both Darby and Gilmore were, it's hard to excel in man coverage when the opposing quarterback can take his time in the pocket. (Peters, by contrast, benefited from a defense that recorded 47 sacks and forced numerous wobbly throws.) With so many defenders injured on the Bills front seven, Darby and Gilmore may spend another season waiting for some pass-rush support.

At least they have each other. "That combination just keeps getting better," McDonald said. "I'm so happy with those two guys."

"I just gotta keep them healthy," the coach added, wary of further angering the injury gods.

Darby earned one more accolade last year: selection to the NFL's All-Under-25 team. It's admittedly a rather obscure honor. "I didn't know too much about that," Darby said. "I guess it was a good accomplishment."

It's also a reminder that Darby is just 22 years old. "Hopefully I can make it for another two or three years," he joked.

Ultimately, individual accomplishments don't matter much to a defender accustomed to sharing glory with famous teammates. It's another typical attitude for a second-year player who knows he still has a lot to learn. "I just want to do good. As long as my team and my coaches are happy, I'm happy."

Darby's team and coaches are happy. That may not be the same for opposing receivers.

Mike Tanier covers the NFL for Bleacher Report. Follow him on Twitter @MikeTanier.