At 24, Will Reiser was diagnosed with a rare form of cancer. Six years later, he’s got a clean bill of health and something else — a movie, based on his cancer experiences. Oh, and it’s a comedy. “50/50,” starring Joseph Gordon-Levitt, opens in theaters Friday; its title comes from the survival odds given to the main character. “It’s fiction,” Reiser emphasized earlier this month, in an interview the day before the film’s premiere at the Toronto International Film Festival. But the movie came from his own “really absurd, surreal experience” of being a cancer patient as a young, single adult. “Everyone else around you who’s sick is much older than you. You feel like a kid.” When he became ill, Reiser was an associate producer on the HBO comedy series “Da Ali G Show,” alongside Seth Rogen. “Seth was my closest friend at the time, and he pretty much went through it with me,” Reiser said of the experience, which included complex surgery to remove a large tumor from his spine, followed by a long and painful recovery. “Our way of dealing with it was that we would make jokes, we would try to find the humor in it.” Friends would ask him if the cancer inspired him to do all the things he’d never done, like a bucket list. “I’d say to Seth, all I want to do is go home and go to bed. I’m sick, I don’t want to go to Africa!” he said, laughing. When Reiser was healthy again, Rogen urged him to write, and a realistic comedy with elements of drama emerged. Now Reiser can watch his screen alter ego go through an experience firmly in his past (he’s six years in remission, and “everything seems good”) and he’s made enough money to pay off the last of his lingering medical bills.