Though he was equal parts actor, writer, and director, for Orson Welles, appearing in his films was simply another way to maintain control over the creative process.
His most notable role, of course, is the one that kickstarted his career: Charles Foster Kane, a media magnate who rose from the ashes then lost it all. Welles was just 26 years old when he made the film that has topped every "Best Film Ever" list in the past 70 years. Ironically enough, the film's unprecedented success way in many ways Welles' own undoing.
Unable to match this original achievement, Welles eventually had to resort to taking whatever crappy acting role came his way, including voiceovers for Magnum P.I. and The Transformers: The Movie.