In Walk Hard: The Dewey Cox Story, the new Judd Apatow–produced spoof on rock-star biopics, The Office's Jenna Fischer plays a winsome backup singer and love interest to John C. Reilly's titular Johnny Cash knockoff. But during the making of the film, Fischer says, it wasn't Cox she had to worry about — it was his ass.

"John has a scene where he has to wear a little diaper and run around. I didn't know that was his wardrobe, number one, but, number two — I didn't even know he was going to be on set that day," she told Vulture in an interview this week. "So they called me up to do my bit, and I go to get in the van that takes us up to set, and there's just this naked ass bent over in the door. And I'm like 'Oh, my God, I picked the crazy van. I picked the van with some insane extra who, for whatever reason, isn't wearing clothes and appears to be struggling either to get in or out of the van.' So I just stood there sort of pretending to read my lines while this person's ass is right in front of me. And then they all started laughing inside the van, because of course it was John. In his diaper."