Hathaway hooks up with her Brokeback Mountain co-star, who plays a Viagra salesman, in the new film.
But the actress insists she's nothing like her free-spirited character, and was an emotional mess when it came time to bare all for the racy scenes.
She tells America's Vogue magazine, "These (characters) are people who have no trouble taking their clothes off - in a way, their bodies are their currency. But they're terrified of exposing their vulnerability, of becoming emotionally naked.
"I was a wreck from start to finish. I think I cried every single day. I had to lean so much more heavily on everyone around me than I'm used to. I'm used to pulling my own weight. But I totally fell apart."