Most divers spend their first fifty dives identifying species and their next fifty watching behaviour, and the second half is where the interest actually is. You do not need a marine biology degree - four patterns cover most of what you will see.
Fish facing into the current
Almost every fish holding station is pointing into the flow, because that is where food arrives from. This is also how you read the current before you leave the boat: look down at the fish.
A cleaning station
A cluster of small wrasse in one spot with larger fish queueing, holding still with their mouths open and gills flared. Approach slowly, stop three metres out, and stay. It is the single most reliable place to watch big fish behave as if you are not there.
Damselfish charging you
A five-centimetre fish attacking a diver is guarding eggs. It is defending a patch about a metre across; back off that far and it stops. It is also a reliable sign you are too close to the coral.
Everything suddenly gone
A reef that empties in three seconds has seen a predator - and you are usually the last to notice. Look out into the blue rather than at the reef.
What this does for photography
Behaviour is the difference between a fish portrait and a photograph. Find the cleaning station, get your settings right on an empty frame, and wait.