The only things I know for sure is thet Duvall would not reprise his role for financial reasons, and Wynona Ryder, who was supposed to play Mary got sick, could not fufill her contract and was replaced by Sofia Copppola.

IMHO, FFC and even Puzo did III for the money more than out of artistic inspiration. I think everyone enjoyed being reunited, going back to Sicily, and enjoying themselves offscreen, but thats a far cry from the effort that went into I and II.

In one of the documentaries about the Godfather, FFC and Pacino make a big fuss about Michael's speech to the corpse of Don Tomassino as representing the theme of the movie, but even that very good scene seems forced, and almost not a part of the picture.

I have no clue what possesed FFC to make some of the awful casting decisions. I think Eli Wallach was some kind of Lee Strassberg protoge, so maybe that had something to do with it, but then again he was also in a bunch of Spaghetti Westerns, which is a sketchy connection. As for Hamilton, I can only guess it was hubris. FFC pulled off using Troy Donahue as Merle in II, but that was a very small part. Hamilton was simply not credible as Hagen's replacement. Even worse, he wasn't even given good lines. Telling the archbishop he got gray hair from worrying was embarrasing.

Even Andy Garcia, who was considered an up and comer at the time was something of a bust, because after that role he really didnt do anything notable until the Ocean's 11, 12, and 13 movies, in which he was already playing a parody of himself.