The transformation of Michael is just amazing to watch, going from “That’s my family, Kay, it’s not me.” to “My father is no different than any other powerful man — any man who’s responsible for other people, like a Senator or President.”
"Now who's being naive, Kay?"
On YouTube there is a video on Francis problems casting Al as Michael. If you see the early screen tests - Al is really wimpy. He and Kay have a chemistry but he looks like a lightweight. The transformation from his screen tests to the end of the movie were light and day.
Francis repeatedly said he always saw Al as Michael. He said Dianne Keaton said Al and George Lucas wife who did the editing said cast Al.
Francis went through even worse misery with the studio, Brando and the weather in Apoc Now. It is a miracle he got that film done.