It's pretty well-known that Seahawks Coach Pete Carroll and GM John Schneider admire taller, more physical cornerbacks. There seems to be a very specific set of criteria the Hawks front office uses when selecting their cornerbacks, and it's one they've stuck to for the most part. I'll try and lay that out a little bit here and attempt to give you an idea of why they hold that belief.
The first, and most obvious point that I'll make is this: a lot of wide receivers are TALL. If you have a very tall wide receiver being marked by a short cornerback, what does the opposing offense do with the football? They throw it high, and the tall guy will catch it. That was from my book, "Football Strategy for People Who Can't Think Good," you can buy it on Amazon. Just picture a shooting guard trying to defend Shaq. He's going to have the quicks to pick off the entry pass to the post once in a while, but for the most part, he's going to be dominated physically because his teammates will simply throw him the ball where the short defender cannot reach it.
I'll name some of the most dominant receivers in the league: Larry Fitzgerald, Andre Johnson, Brandon Marshall, Calvin Johnson, Vincent Jackson, Sidney Rice, Marques Colston, .. the list goes on. All over 6'2, 215 lbs. Remember when Dwayne Bowe came to Qwest last season? 13 receptions, 130 yards, 3 TDs. We want to try and pare that down a little bit moving forward. Now, obviously some really good receivers are smaller but on a whole I think the NFL is moving towards taller, longer, more physical receivers as the NFL becomes more and more a passing league. John Schneider and Pete Carroll have both stated that they are trying to get taller at the cornerback position for this specific and conspicuous reason.
Apart from that though, there are some other physical attributes they look for in a cornerback. First, his tackling ability. Pete Carroll tends to run a lot of two-high zone coverage, or he'd like to anyway, to pair with his 4-3 Under defensive front. Because the Cover-2 is often used in conjunction with the 4-3 Under 7-man front, certain types of players are useful. Strong tacklers are important because in this defense they are often the spill containment. As Jene Bremel put it, "the corners are often the "help" when the front seven can't make the tackle. The play-side corner gets the bulk of the extra business - often the strongside corner - but both corners may be the force player in run support as often as the safety."
With the Cover-2, the cornerbacks are responsible for the "flats", meaning they don't have to play bump-and-run with the opposing WR after that receiver has left their zone, which extends about 15 yards or so from the line of scrimmage. On run plays, because of this, they're able to disengage from their mark much earlier knowing there is help over the top from the safeties in case the running back decides to chuck the ball downfield.
The other attribute to look for is physicality. In certain schemes, Carroll has his corners up in the opposing wide receivers' grills, and in others, they'll be backed off 5-10 yards. It really depends, but in situations where the corner is playing press-coverage at the line with the wide-receiver, he has to be strong enough to control leverage and force the opposing receiver to the side that he wants. As Pete Carroll said, "No matter what coverage you are playing you have to convince your players to win their leverage side. If the coach tells a player to play outside leverage and complains when a receiver catches a ball to his inside, the coach is wrong. When we give them a leverage side, we are telling them to just do that aspect right, at least."
So in other words, if a player is too weak to win the leverage battle, he's going to cause major problems for the defense. Result? They want strong, physical players.
In a Cover-1/Cover-3 situation (both schemes the Hawks run a lot), Carroll will often ask his players to use inside leverage (he did at USC anyway). As he put it, "we tell our corners to play inside leverage (i.e. to the inside shoulder of the receiver) in this defense. This helps the corner avoid giving up the big play to the inside of the field. If you want them to play the out route towards the sideline you have to give them someone playing support over the top. There is not a corner in college or the NFL that can both play the out routes and also avoid giving up the deep ball to the inside. You have to be realistic as to what your players can do. They only way a corner can play inside leverage and make a play on the out route is if the offense screws up or the quarterback makes a bad throw or the receiver runs a bad route. If you don't understand that then you are asking the corner to do something he can't do."
In other words, it's pretty tough to prevent against giving up the out-route AND the big-play post route when the corner is on an island one-on-one with a receiver. At least, with the out-route, it's a tougher throw to make for the quarterback and his receiver has to run a very good route. Also, because our corners are tall, it makes it that much tougher to put the pass on the money out of the outreached arms of the corner playing inside leverage. Furthermore, if our corner is good on the jam and can slow up the receiver it will affect the timing of the out-route, a play that is pretty precision-oriented as the wide receiver runs full speed toward the sideline. If that timing is disrupted and the throw comes early or late, it can big a good stop or even a turnover.
Not every corner is going to Nnamdi Asomugha or Darrelle Revis and be able to guard everything - you have to be honest with yourself on some things and prevent the big play over the top. Hence, the 'bend-but-don't-break' defense. Some people hate this, but it's something we're forced to watch because I don't see Pete Carroll changing his philosophy anytime soon. Just in general, it's very, very hard to play defense on a guy that is running full speed and knows where he is going while you sit there in your backpedal and have to guess. Instead, focus on getting yourself a player that is big and strong, can disrupt routes and get his arms into passing lanes.
The final piece of the puzzle is a corner's speed. Most of the corners that we have are pretty fast. 4.4-4.5 type guys. To get yourself a big physical corner that can run a 4.3 you have to be picking in the top 10, typically. Of course, Jimmy Smith was an exception but the character concerns were there. We chose Walter Thurmond last year in the fourth round but he's arguably second round talent - he's got the speed, and he's tall,- less physical- but excellent in coverage. This season, we chose Richard Sherman and Byron Maxwell and both are tall, physical, and fast. They're both raw of course, but you see a plan emerging. In the Seahawks' case, Thurmond projects to be a coverage CB with ability to run with anyone. Sherman is taller and longer but a bit slower, and projects as a player that can guard the bigger receivers we face. Maxwell is a little bit of both, but his speed and physicality make him quite intriguing.
The type of coverage called is somewhat dependent on the opposing personnel. If you are facing off against a team with smaller, speedy receivers it makes sense to run zone coverages with help over the top. From Carroll, "The corners have to run fast if you plan on playing bump and run. If they don't run fast then you can still play with them. But if your corners are not faster than the wide receivers you are facing don't play bump and run. You're asking them to do something they can not do and they'll get beat deep. It is a race when you play bump and run and if you can't win the race don't play bump and run."
In other words, if you're playing a team like Philadelphia, for instance, that utilizes more small, agile, and speedy receivers like DeSean Jackson and Jeremy Maclin, it makes sense to run a zone if you don't have elite speed at the corner position. It's really just the use of common sense here: if your corners are slower than the opposing receivers, don't ask them to play man-to-man because they'll lose the race. Instead, play a zone, get physical at the line with your big corners, disrupt routes, manhandle at the line, and have protection over the top. Like in the Tampa-2, or a Cover-2.
Anyway, there are too many wrinkles and caveats for NFL player selection to go much deeper than this, but hopefully I've given you a picture of the plan. I'll continue on with this series soon and go through the other defensive positions.