Though Michael J. Fox was indeed the first choice to play Marty McFly, his commitments to TV show Family Ties prevented him from initially taking the part, which eventually went to on-the-rise actor Eric Stoltz instead.
After a whole four weeks of shooting, director Robert Zemeckis decided that Stoltz was miscast, playing the part too seriously and lacking the laid-back charm necessary for the character.
At a cost of $3 million, Zemeckis decided to start over, offering the role to Fox once again, who now had a free enough schedule to take the part.
The rest, of course, is history. It's Fox's signature role and it's hard to imagine anyone else pulling it off as well as he did.