In this famous “Mouth of Truth” scene, Roman Holiday movie,

Gregory Peck ad-libbed the joke
where he pretends his hand gets bitten off in the mouth of the stone carving.
He borrowed the gag from Red Skelton. Before shooting Peck told the director
that he was going to do the gag but did 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.