I have to say the scene between Michael and Fredo is the best and both give great monlogues in it.

I really dont think he overdid it either DonSonny. I mean Fredo had been a part of the family business a lot longer than Michael (even tho he never really did anything important) so it had to have angered him how Mike was able to just come in out of nowhere and become the new Don Corleone. Like he said, he was "passed over" and had never vented out his anger to anyone. He's suppose to look like he has a cerebral pulsy cuz he has no control of his emotions in that scene. It was a great acting job by Cazale.