Remember back when we had actors like Charlton Heston? Who'd refuse to do sequels on principle, feeling that ultimately they could never be quite as good as their predecessor.

This is why he had such a small role in Beneath the Planet of the Apes. He didn't want to do the film at all, but since they couldn't do it without him, he offered to do it if they would kill off his character in the first ten minutes. They agreed, and later asked him if they could modify the first ten minutes thing in order to have him killed in the climax, but still write it in such a way that he still only has about ten minutes or so screen time, which obviously he agreed to. He also suggested they blow up the planet at the end of the film, and was happy when they decided to go with that, because he thought that would mean there couldn't be any more sequels at all... :D

Anyway, too bad more actors don't take this kind of approach and pride to their acting careers. Instead, they have no problem doing the same thing over, and over, and over, and over, and over, and over, and over... so long as they keep getting paid for it.