I like Robert Duvall, he's a good actor and should have won an Oscar for Apocalypse Now. He was good as a consigliere, but he was also supposed to be a former street kid who was adopted and raised by that family, it didn't seem believable.
He was supposed to be a good kid who was saved by Sonny from an abusive alcoholic father. It explains his intense loyalty to Sonny and his anger after Sonny got killed.