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Michael J. Fox was the original choice for Marty but was unavailable due to his commitments on Family Ties. The studio's desperation led to an extraordinary and somewhat chaotic arrangement: Fox would work on Family Ties during the week, then immediately switch to filming Back to the Future on weekends and late nights.
This grueling schedule meant Fox was often running on only two hours of sleep per night for three months. His dedication ultimately paid off, bringing the movie to life with the charm and comedic timing the role required.