Epic fail

Found it, wiki;

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.[4]