Richard Stanley was hired to direct “The Island of Doctor Moreau” but was replaced by John Frankenheimer after a few days of shooting. However, Stanley considered the film to be his baby (he co-wrote the script) and didn’t want to leave. So he disguised himself as one of the mutants and secretly remained on the shoot.
You can watch the documentary about the shooting of this B-movie and it’s full of weird details like that. It’s called “Lost Soul: the doomed journey of Richard Stanley’s Island of Doctor Moreau”