Fussy 3-year-old
The little boy who played Michael Corleone's son is named Anthony. He refused to be called anything other than his real name, so they had to change it in the script. Luckily, Anthony is already an Italian name!
The Book
Mario Puzo is the author of the original Godfather book. Obviously, the filmmakers had to cut out a lot of the story in order to make it movie-friendly. Some relationships were left out, including some weirdness between Sonny and his mistress. It's probably for the better!