John Cazale died in 1978 aged only 42 of cancer
I think he was in only five films all of which were acclaimed and won several awards though sadly he was not personally nominated, in spite of his stellar performances

The confusing and unanswered ambiguity were saved by the superb acting including,
1. who killed the Tahoe assassins
2. was Fredo's little help, opening the drapes, for the Donship
3. why did Michael tell Fredo that Hyman Roth will never see the New Year
4. how did Fredo get out of Cuba
5. how did Kay get the abortion
6. why did Michael just accept the abortion

Biggest question of them all
Why didn't Coppola pay fair money to Robert Duvall to reprise Tom and make a decent GFIII