Michael J. Fox was the first choice to play 
Marty McFly, but he was committed to the show 
Family Ties.
[11] Family Ties producer 
Gary David Goldberg felt that Fox was essential to the show's success. With co-star 
Meredith Baxter on 
maternity leave, he refused to allow Fox time off to work on a film. 
Back to the Future was originally scheduled for a May 1985 release and it was late 1984 when it was learned that Fox would be unable to star in the film.
[6] Zemeckis' next two choices were 
C. Thomas Howell and 
Eric Stoltz. Eric Stoltz impressed the producers enough with his earlier portrayal of Roy L. Dennis in 
Mask – which had yet to be released – that they selected him to play Marty McFly.
[4] Because of the difficult casting process, the start date was pushed back twice.
[12]
Four weeks into filming, Zemeckis determined Stoltz had been 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.
[6]
Fox's schedule was opened up in January 1985 when Meredith Baxter returned to 
Family Ties following her pregnancy. The 
Back to the Future crew met with Goldberg again, who made a deal that Fox's main priority would be 
Family Ties, and if a scheduling conflict arose, "we win". Fox loved the script and was impressed by Zemeckis and Gale's sensitivity in releasing Stoltz, because they nevertheless "spoke very highly of him".
[6] Per Welinder and Bob Schmelzer assisted on the skateboarding scenes.
[13] Fox found his portrayal of Marty McFly to be very personal. "All I did in high school was skateboard, chase girls and play in bands. I even dreamed of becoming a rock star."
[11]