Did you know that James Cameron was in the drawing scene in Titanic? Those are his hands doing the actual drawing. Yes, the tasteful PG-13 nude was by Mr. Avatar himself.
Mel Gibson also got his hands metaphorically dirty with an appearance in The Passion Of The Christ, when he decided that he should be the one driving the nails into Jesus’ crucified palms. His line for the scene, “Hands up, Jesus!” was left on the cutting room floor.
Ridley Scott is another director who sneakily got his frisky digits an IMDb credit, in this case in a scene in Alien. It was Scott’s hands creating the fluttery movement in the alien egg, right before John Hurt gets his face impregnated. And once you know that the effect was simply Sir Ridley making bird-shaped shadow puppets in a pair of dishwashing gloves, you’ll never unsee it.