Cera planted his friend in the audience and had him ask a fake question as a prank.
"I said to him, 'Okay, raise your hand to ask a question, and if they call on you, say, 'I have a two-part question for Michael.' "
"'The first part is, 'When is the last time you saw your son Amos,' and the second part is, 'If you're so sure he's not your son, why won't you take the DNA test?' " (Obviously, Cera does not have a son.)
Cera said that there was complete silence in the room after his friend asked the hoax question, and after he faux-brushed the question aside, the crowd turned on his friend. Letterman, for his part, said Cera should be banned from Sundance for life.
The film in question, Crystal Fairy & the Magical Cactus, follows Cera and some friends as they search for a legendary hallucinogen – it's currently out on DVD. Watch the full clip from Cera's interview below.
Ilya S. Savenok / Getty