While we're talking about supporting characters, I've just got to say that I've heard the race track tout played by both Benny Rubin and Sheldon Leonard, and although the part is exactly the same, it just doesn't really *sparkle* until Leonard takes it over. Once he's got it, he has the absolute perfect "gangster" voice to deliver the lines. Rubin is a great character actor, but I don't know, he just sounds like he's too forced in the role compared to Leonard.
Whenever I see the original Star Trek episode "A Piece of the Action", I always want so much to see Sheldon Leonard come out and tout Captain Kirk to do something or other. I know it will never happen, but some fantasy casting/scriptwriting there.