Hayden Cristensen

Many directors cast unknown actors in films to make sure that the audience's impression of the character is not tainted by the knowledge of the actor's previous roles. This is said to be a key reason why the Nick Cage "Superman" film was never made.

However George Lucas seems to have gone one better. In casting Hayden Christensen in the lead role in two of the biggest grossing films of all time, Star Wars II and III, he has managed to cast an actor who is not even famous AFTER appearing in the films.