"I knew there was a scene in which I had to get naked," Cooper, 30, told PEOPLE at Wednesday's special Chanel and Vogue screening of the film in New York for the Cinema Society. "It wasn't gratuitous, but still quite overwhelming. There were a series of devices I was offered to wear which protect my [naughty] bits. And I gave Keira the choice in a very gentlemanly way, 'You can chose either the furry soft, the pink diaper or the Spandex."
Although Cooper, who was also seen (though not in the altogether) in Mama Mia, considered all three "horrifying"], when it came to Knightley "I showed her the devices, offered my opinion and she chose the diaper."
Why the diaper? "Because she wanted to humiliate me," insists Cooper. "She laughed and laughed and laughed. That's what made the scene quite comfortable [to shoot]. We realized how ridiculous it was."
He Said, She SaidThat, however, isn't quite how Knightley, 23, recalls the filming and the specific costume prop.
"I don't remember choosing," she tells PEOPLE. "I think he's making that up. Unless I was feeling particularly malicious, I don't think I would have chosen to put a man in a skin-colored diaper. I could have been feeling particularly malicious, however, that's entirely possible."
As for how Cooper looked in – and out – of the diaper, "He's a good-looking guy," said Knightley.
Added Saul Dibb, director of the movie, which also stars Ralph Fiennes: "It's always the women who are naked. In this one, the men are naked, largely."
2004: Keira kicks the boys of Arthur's butts!