Coaching, when you get down to the point, is fairly simplistic as far as playcalling goes, even moreso for defensive coordinators. You have several sets you use, 1 or 2 that you use the majority of the time, and you have different variations of what certain players do in those sets. Defensive playcalling is, a lot of the time, guessing what the offense will do and hope that you guessed right. Obviously, certain situations dictate certain formations, like when the offense is on the goal line or when they are throwing deep with 3 seconds left, but that is not the majority of the time.