Michael J. Fox had always been the first choice for Marty, but he was unavailable due to scheduling conflicts with his work on Family Ties. As “Family Ties” co-star Meredith Baxter was pregnant at the time, Fox was carrying a lot more of the show than usual. The show’s producer Gary David Goldberg simply couldn’t afford to let Fox go. Robert Zemeckis and Bob Gale then cast Eric Stoltz as Marty based on his performance in Mask. After four weeks of filming Zemeckis and Gale felt that Stoltz wasn’t right for the part and Stoltz agreed. By this stage, Baxter was back fully on the show and Goldberg agreed to let Fox go off to make the film. Fox worked out a schedule to fulfill his commitment to both projects. Every day during production, he drove straight to the movie set after taping of the show was finished every day and averaged about five hours of sleep. The bulk of the production was filmed from 6pm to 6am, with the daylight scenes filmed on weekends. Fox found it exhausting, but “it was my dream to be in the film and television business, although I didn’t know I’d be in them simultaneously. It was just this weird ride and I got on.” Zemeckis concurred, dubbing Back to the Future “the film that would not wrap.” He recalled that because they shot night after night, he was always “half asleep” and the “fattest, most out-of-shape and sick I ever was.”

(Source: filmtrivia)