Was looking up Robert Duvall's page on IMDB, and found this, in reference to GF3:

[on the reason he didn't appear in The Godfather Part III] "If they paid Pacino twice what they paid me, that's fine, but not three or four times, which is what they did. (Francis Ford Coppola) came to my farm, parked his car ... went in the kitchen. (I) said: 'I know you always wanted the crab cake recipe, let me cook it for you.' Oh, he loves to eat, so I cooked the crab cake... and he wrote it down ... and he forgot it, so he called twice. He was ... more concerned that he forgot the crab cake recipe than would I be in Godfather III." [January 8, 2004]

LOL, I hope that last bit was a joke...