It is NOT based on approval at all. I have very high approval with Astarion and neutral with everyone else (never use any of them) and after 3 times playing through the game I still didn't get nearly half the camp cutscenes with him... it seems everyone is overriding him. It is beyond frustrating that I'm forced to NOT recruit anyone else so that I can see all the scenes with the only companion I'm interested in. I really hope it will get fixed soon. Dragon age implemented it rather well, aside from the cutscenes you also get to hear an input to any important moments in the game from all the companions in the party.
