Eric Stoltz as Marty McFly
Four weeks into filming, Zemeckis decided Stoltz was miscast. Although he and Spielberg realized reshooting the film would add $3 million to the $14 million budget, they decided to recast. Spielberg explained Zemeckis felt Stoltz was too humorless and gave a "terrifically dramatic performance". Gale further explained they felt Stoltz was simply acting out the role, whereas Fox himself had a personality like Marty McFly. He felt Stoltz was uncomfortable riding a skateboard, whereas Fox was not. Stoltz confessed to director Peter Bogdanovich during a phone call, two weeks into the shoot, that he was unsure of Zemeckis and Gale's direction, and concurred that he was wrong for the role.
Way back when Back To The Future was just starting, Michael J. Fox was working on a comedy show Family Ties. He'd heard about "that Future thing" Stephen Spielberg was doing, and had been their first choice for the role of Marty. But Gary Goldberg, the producer of Family Ties didn't tell Michael, because it was impossible to let him out of his Family Ties responsibilities and he didn't want to disappoint him. So, with their first choice down the drain, they cast Eric Stoltz:
About two months into filming, they discovered that as good an actor as Eric was, he just wasn't right for the role of Marty. In a pretty uncharacteristic move, they decided they were prepared to recast the part and reshoot all his scenes (which, being the star of the movie, meant practically every scene filmed so far), a very expensive thing to do. Again they pushed Gary to let Michael participate, and while he still couldn't let him out of his Family Ties commitments, he didn't want to rob him of this chance again. Michael J. Fox was cast as Marty McFly, and worked all day on Family Ties and all night on Back To The Future.
It's said the Stoltz footage still exists somewhere, but the directors decided not to include it in the DVD box sets as they didn't want to "contaminate" the movies as they are. Maybe we'll see it in the future.
Goes to show that casting the right person does make a difference. If they were able to go with Fox to begin with they wouldn't have cast Lea Thompson as Lorraine McFly. (She got the role because she had acted opposite Stoltz in The Wild Life.) Or Thomas F. Wilson as Biff Tannen because the original choice, J. J. Cohen, was considered too unconvincing to bully Stoltz. IMO Thompson and Wilson were two weaker choices who did not hold up well in the sequels.
