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.
People might think of Jon Voight now and laugh but thinking of how he was in the 70's, he could have played Charlie well. A young, brash man who had Charlie's charisma, like Keitel. The advantage I think Keitel had however was growing up in Brooklyn, as opposed to Voight having grown up in White Planes. Keitel just got it in a way that I don't think Voight would have.
