In Time Bandits, in the scene introducing Agamemnon, the script read: Helmet comes off to reveal Sean Connery (or an actor of equal but cheaper stature).
Imagine, if you will, that the casting director hired Roger Moore.
Later: I learned last night from some website or other that Franz Liebkind, author of Springtime for Hitler, was almost played by Dustin Hoffman (who did The Graduate instead).