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.
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.
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.
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.
In setting up the sex scenes, the actors were encouraged to improvise, but with their clothes on. Edward Zwick says that it is "colder" on a set because there are people all around, the camera, and other distractions. Therefore, the improvisation was useful in setting the scenes and shots for the actual shoot.