For whatever reason, be it a deep-seated desire to act, a lack of a casting budget, or just “if you want something done right, do it yourself”, many directors at some point in their careers have stepped out from behind the camera to act. This is typically in a smaller, cameo role, and often with varying degrees of success: sometimes they’re completely natural and sometimes they bring the film to a screeching halt. And sometimes you’d never even know they were there. (Cineleet)

"Arnold Schwarzenegger will be invited to take a cameo role in a fourth Terminator movie. Producers want the Governor of California to make an appearance in the film. (Terminator files)

Julie played a cameo role in Harry Potter and the Prisoner of Azkaban (2004), playing Madame Rosmerta. (BBC Home)