James McAvoy was cast as Adam, but had to drop out due to personal conflicts. Joseph Gordon-Levitt replaced him after being called by Seth Rogen less than a week before shooting was scheduled to start. He accepted the role just two days before.
Adam is based on writer Will Reiser, who was diagnosed with cancer and later recovered. Seth Rogen, who plays Kyle, helped Reiser cope with his disease and convinced him to write a screenplay during their early 20s together.
During a roundtable discussion with the cast, screenwriter Will Reiser revealed that Bryce Dallas Howard came up with the title of the film. The crew was skeptical about it when she first suggested it to them on the set of the art gallery scene. When she found out that the title had been changed to the one she suggested, she was surprised.
While being interviewed by Terry Gross on her National Public Radio program "Fresh Air," Will Reiser said that he gave Adam's character a job at a Seattle NPR station because Reiser is a big fan of NPR, but he had not ever donated any money to any of their pledge drives.
50/50 (2011) is Joseph Gordon-Levitt's second film beginning with the numbers 5-0. He previously starred in (500) Days of Summer (2009) in 2009. Levitt was nominated for the Golden Globe as Best Actor in a Comedy film for both movies, and both films earned a second nomination as Best Comedy at the same award.
The trivia items below may give away important plot points.
Summit Entertainment wanted a kiss between Adam (Joseph Gordon-Levitt) and Kathryn (Anna Kendrick) at the end of the film. However, the actors and crew agreed that it would not feel right since they felt the movie was about the main character's journey with cancer, rather than a love story. They almost filmed the scene to appease the studio, but Levitt pointed out that "If they gave the studio the option, they would use it," so they just refused to film it.