I just saw the movie 25th Hour the other day, and I have to say I absolutely DO buy Edward Norton as a possible Fredo. Norton in Primal Fear, also, has that kind of squirrelly mannerisms that are very characteristic of Fredo.

I don't know what it was, but throughout the movie 25th Hour, Norton's look was very reminiscent of Fredo. I'm sure with a good hair dye/grooming job, he would have a good resemblance to Cazale.

Remember in all this talk about casting ideas: it's not neccesary that a casting choice has to be a dead ringer for the character. It just has to be close enough that it doesn't visually throw you off - close enough that you can believe that this guy is Fredo, Michael, Sonny, Vito, etc.

If anyone's got any BETTER ideas, fine, but right now, Norton is the one actor I can think of who could fill the bill for Fredo.