A couple of useless (but hopefully interesting) bits of casting trivia:
Ernest Borgnine read for the part of Vito Corleone. Brando was great in the part, but I think Ernie would have been quite good, too. Borgnine is, IMHO, a far better actor than he often gets credit for.
When casting the part of that little weasel Carlo (Gianni Russo), one of the actors considered was none other than Alex Karras. I don’t think that would have worked too well. At least the fight seen between him and Caan wouldn’t have worked out. I mean, James Caan is tough, but *ain’t* whipping Mongo! :-)
BTW, one often wonders what John Cazale might have done had he not died at such a young age (bone cancer took him at just 42). He was every bit the acting equal of Pacino and DeNiro in Godfather/GF II, IMHO. He was also outstanding in the few other films he was able to complete before his death (Deer Hunter, Dog Day Afternoon, and the oft-overlooked The Conversation).