Using Improv
James Caan was big on improvising his lines. The scene where he used a carbage can lid to beat up Carlo was totally on his own accord. He used the phrase "bada-bing!" completely on his own because he heard mobster Carmine Persico say it once. Of course, the famous scene where he throws the FBI photographer to the ground was also totally Caan!
Brando's Improv
Brando also improvised some lines and actions! The scene where Vito smacks Johnny Fontange in the face was not in the script. That's why Johnny's reaction was so real!