Marlon Brando Was The Real Top G

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The smack that Marlon Brando gives Al Martino was not in the script. Marlon Brando improvised the smack and Al Martino's confused reaction was real. James Caan said that Martino didn't know whether to laugh or cry.
Furthermore, one of the godfather producers said that Marlon Brando did not like Al Martino at all. So, that might've been a bit personal.