In the last few scenes of the movie you see the Attorney General (he's the man with the cowboy hat and round sunglasses) this man is also the Stunt director/organiser. He contributed a lot to the film as all the stunts where organised by him so the director decided to give him a small part as a way of saying thank-you.
The onions that Stanley and Zero eat towards the end of the film are actually apples with an edible covering.
Sigourney Weaver took on the role of the Warden at the suggestion of her daughter. Holes is her daughter's favourite book.