Do we know if the storyline/script was written when Duvall was approached? Was he turning down the role that Hamiliton had? If so, that was a small part. I would venture to guess that if Duvall was in GF3 that the role would have been bigger.

It's hard for us peon's to know what goes on the mind of the actor. Are they secure in their vocation. Is every film opportunity looked on as their last? am I now only good enough for B movies? or Hey, I'm better than that! I don't think we the viewer can be too judgemental about the psychology of the movie industry and the personalities that we know little about.