The movie filmed for 3-5 weeks with Eric Stoltz as the star. According to screenwriter Bob Gale, "There was about 40 minutes worth of cut footage." Unfortunately, Eric Stoltz was simply miscast. "And he is intense, and he’s more of the method school of acting, and he’s a very internal actor, as opposed to a guy who has a lot of physicality to him." said, Bob Gale.
In an interview with BoxOfficeMojo Bob Gale explains, "Gale: Bob [Zemeckis] was cutting the movie as he shot it, and he said to [producer] Neil Canton and me, after he screened forty minutes of cut footage [with Stoltz as Marty]: "I think we've got a problem—I want you guys to look at it and tell me what you think." So we looked at it and said, yeah. Then he screened it for [producers] Frank Marshall and Kathy Kennedy and later Steven Spielberg and everybody agreed it just wasn't working. We had wanted Michael J. Fox to begin with but [Fox was committed to playing Alex P. Keaton on NBC's Family Ties]
BOM: Who made the call to Eric Stoltz?
Gale: Bob Z. He just sat Eric down in the trailer and gave him the bad news."
Which probably isn't the worst news you could hear in a trailer, but still. What's the worst you've heard?
Ms. Baxter Birney (The Mom) was back to work at Family Ties, and the more animated Michael J. Fox was brought in to replace Eric Stoltz. The Robert Zemeckis production went back in time and the 22-year old shot around Family Ties' schedule. The bold move paid off for all involved and Stoltz has no regrets (publicly). Though I bet he's a bit wary of trailers.
The major visual differences between the two Marty McFlys involve wardrobe. Stoltz's McFly tried to get by wearing two layers (T-Shirt & Jacket), which was obviously not enough, because Michael J. Fox's McFly stacks on four layers (T-Shirt, Long Sleeve Shirt, Jeans Jacket, Vest). The more layers you wear, the warmer your performance. Also, Michael J. Fox looks infinitely more confused than Eric Stoltz. Perhaps some day we'll get to see and hear Eric Stoltz's 40 min. serious interpretation of Back to the Future.