At one point, director Edward Zwick jumped into bed with Anne Hathaway and Jake Gyllenhaal naked for a group shot to make them feel more comfortable. This shot was used for the film's poster, with Zwick digitally removed.
In preparation for their respective roles, Jake Gyllenhaal spent time with Jamie Reidy, the author of the book the film is based on, while Anne Hathaway spent time with Lucy Roucis, an actress stricken with Parkinson's disease, who has a role in the film.
Anne Hathaway lost weight for the role not because she was required to frequently be nude but because she'd read that many patients when they're first diagnosed with Parkinson's often shed weight.
When asked about the sex scenes, Jake Gyllenhaal joked: "I've had a pretty illustrious career at this point, but the hardest thing has been pretending to be turned on by Anne Hathaway. And I pride myself on really having pulled it off."
The anti-Parkinson's drugs Sinemet and Ropinirole mentioned in the film can cause sexual hyperactivity and hyperarousal in some patients as a side effect.