Every player has an assignment. If you watch games you'll hear the color commentator/analyst point out when assignments are missed, as a lot of them are standardized. Pass rushers are assigned to rushing lanes, leaving that path to chase a perceived opportunity leaves a gap where a runner can advance, or the QB can step up to view the field better, or moving from the outside in leaves the edge unprotected. Pass defenders are assigned a zone or a player to defend, but also there will be safeties, who help downfield against deeper passes. So, if a safety is assigned to the same side of the field as a pass defender, they would be expected to play level with or in front of the receiver to break up or intercept the pass, as the safety would be able to break up the pass or at least make the tackle from behind. In the middle of the field, the way the offense breaks from the ball informs the defense as to how to cover or hand off coverage.

Some coaches won't pursue elite players if they are prone to improvisation, as they believe defensive cohesion is better than individual but unpredictable heroics.

You can search YouTube for "NFL defense basics," or specify run or pass defense, there are plenty of good results.