I just read an interesting interview that Tarantino had with Scorsese. Apparently the studio wanted Jon Voight as Charlie. At one point there was even talk of Keitel stepping aside to play Johnny Boy. Voight ended up turning them down so it was much ado about nothing. BUT both David Proval and Richard Romanus were acting students of Voight's. Scorsese ended up casting them while observing Voight's classes.