missavagardner:

For the famous “Mouth of Truth” scene, Gregory Peck ad-libbed the joke where he pretends that his hand was bitten off in the mouth of the stone carving. He borrowed the gag from Red Skelton. Prior to filming the scene, Peck told director William Wyler that he was going to do the gag, but that they should not tell Audrey Hepburn. When Peck pulled his arm out of the stone carving’s mouth with his hand pulled up his sleeve, Hepburn’s horror and surprise was genuine. She gave what she later recalled was “a good and proper scream,” and the scene was finished in one take.