The 28-year-old actress - who plays young Parkinson`s Disease sufferer Maggie Murdock in `Love & Other Drugs` - did lose weight for her part because she heard the effects of the medication causes people to, but would have bulked up if the role had called for it.
She told BANG Showbiz: "When it came time to decide how the character would look I did a lot of research into the side-effects Parkinson`s medication would have on the human body and I actually found that in the majority of the cases it would cause people to lose weight so that was kind of the jumping off point for how she would look in the film. But it wasn`t resting on that - had the medication caused people to gainweight I would have gainedweight and still done the nudity."
The Oscar-nominated star believes nudity was an essential part of the film because it helped her and co-star Jake Gyllenhaal explore "intimacy".
She added: "I thought the nudity was an essential part of the story and the intimacy Jake and I can feel together and how the relationship shifts from sex into love. I think the film is a really wonderful exploration of intimacy in a relationship."
`Love & Other Drugs` comes to UK cinemas on December 29.