Nobody could have played Fredo better than Cazale.
Shame he died so young, right after The Deer Hunter, I believe.
HBO did a retrospective on Cazale’s movie career. His health was so bad during the filming of the Deer Hunter that the studio could not insure Cazale and was going to use another actor for the part.
I believe that it the was DeNiro who came up with the money to insure Cazale that allowed Cazale to be in that movie.
Cazale either died before filming ended or right after. It was amazing that he was able to make that movie.