Two years were needed to finish building the sets and writing the script before shooting could begin. During filming, the creation of the appearance of the aged characters was a well-guarded secret, involving state-of-the-art make-up techniques. Fox described the process as very time-consuming. "It took over four hours, although it could be worse". Principal photography began on February 20, 1989. For a three-week period nearing the conclusion of the film, the crew split and, while most remained dedicated to shooting "Part III", a few, including Gale, focused on finishing its predecessor. Zemeckis himself slept only a few hours per day, supervising both films, having to fly between Burbank, where it was being finished, and other locations in California for "Part III".