The studio was constantly trying to dump Pacino. Supposedly Coppola shot the film from the end to the beginning to insure Al was not dropped. Pacino early on was not confident but as the shooting went on he found his feet. I think Hollywood does glorify the mafia but the characters, the acting, the story and the production were outstanding.
The first 2 Godfathers, Apoc Now and The Conversation prove that Coppolla is a very good director.
The studio suggested Robert Redford (YEECH!) for Michael, while Robert Evans (I think) was the one who suggested John Cassavetes (who would have been too old). A damn shame that Pacino has evolved into a screaming, screeching parody of himself since “And Justice for All.”