Found this out today
http://en.wikipedia.org/wiki/Back_to_the_Future
"Eric Stoltz vs. Michael J. Fox
As Back to the Future's producers were scouting locations on a residential street in Pasadena, Michael J. Fox was elsewhere on that street, filming what became his first starring feature role, Teen Wolf. The producers became interested in having Fox play Back to the Future's lead role of "Marty McFly". However, Fox initially had to turn down the part because the producers of the Family Ties television show wouldn't allow Fox's character on that show (Alex Keaton) to be absent from any episodes.
Production of the film began on November 26, 1984 with actor Eric Stoltz portraying Marty McFly. But after filming began, the filmmakers came to the conclusion that Stoltz was not right for the part. Stoltz had played it serious, but they wanted a lighter touch on the character. It is believed that they had, at that point, filmed about one third of the completed movie. They returned to the idea of Michael J. Fox, who this time worked out a shooting schedule that wouldn't interfere with his commitments on Family Ties. Fox would spend his days rehearsing and shooting Family Ties, and then drive immediately over to the movie's set and film Back to The Future all night. The movie's day shots would be filmed on weekends.
Fox reportedly averaged only an hour or two of sleep each night during production. Shooting was completed on April 20, 1985, less than three months before its release.
Footage with Stoltz as Marty McFly still exists, according to Robert Zemeckis and Bob Gale. One notable scene with Stoltz that was kept in the final film is the scene in the mall parking lot in which Marty is driving the time machine. Stoltz is at the wheel of the DeLorean in that scene. After the change in actors occurred, it was decided to stick with the previously filmed footage for that scene, since the shots were fairly distant, with the driver's face not particularly visible.
More detailed still photos featuring Stoltz in the role can be found at BTTF.com."
Links
http://www.bttf.com/cgi-bin/ImageFolio3 ... cFly&img=0
[img]http://www.kristensheley.com/bttf/pics/estoltz.gif[/img]