When Bob Zemeckis and Bob Gale wrote Back To The Future, they had one person in mind to play Marty McFly - Michael J Fox. At the time, Fox was filming the TV sitcom Family Ties and Meredith Baxter-Birney (one of the other stars of Family Ties) was pregnant, so Fox had to carry the show. The Bobs asked the producer (Gary Goldberg) if Fox could be freed to do BTTF, and he said no.
Head of Universal Studios, Sid Sheinberg, asked the Bobs to go with their second choice, Eric Stoltz, and half-jokingly said that if it didn't work out, they could recast and reshoot. Anyway, Eric shot a lot of the movie, but Bob Zemeckis thought that although he was giving a good performance, he just wasn't "wacky" enough, he was too thoughtful.
So, the Bobs went back to Michael J Fox. By this time, Baxter-Birney had had her baby, and so there was less pressure on Fox. The producer agreed to let Fox film BTTF so long as it did not interfere with the Family Ties schedule.
And so, the Bobs salvaged all the footage they had which didn't contain Stoltz, and shot all of Fox's scenes at night, or at weekends - with Fox getting only a few hours sleep before returning to shoot Family Ties.
Eric has been asked about BTTF in interviews, but his terseness suggests that he's not interested in talking about it.
Originally, Melora Hardin was cast as Jennifer Parker. When Eric Stoltz was replaced, it transpired that Hardin was taller than Michael J Fox, which some of the production staff had a problem with, so she was also replaced - by Claudia Wells. Melora didn't get to film a single shot. Since BTTF isn't mentioned once on her webpage, I'm guessing she's not interested in talking about it.
It's not known exactly how much footage was shot of Stoltz, but some details are known
Thanks to images which have been released, and the commentary on the region one DVD, we know that the following scenes were definitely shot:
When a film crew make a movie, they tend to shoot all the scenes set at each location, before moving on to the next location. For this reason, there are various scenes which we can be fairly confident were also shot with Eric:
We can't be sure about the mall car park, because Stoltz was fired while they were shooting here, so we don't know exactly what had been shot. Also, we can't be sure what was filmed at the school since they had to fit their shooting into weekends, and they still had at least some scenes left to film (see below).
We know that Melora Hardin never shot any scenes, and we know that Courthouse Square was not changed to 1985, so based on this and the fact that film crews usually shoot all scenes in one go, we know the following were NOT filmed:
There are three shots in the finished movie which people claim are actually of Eric Stoltz, not Michael J Fox.
The first of these is where Marty dives into the DeLorean, when the terrorists shoot Doc. People think it doesn't look like Fox, and assume that it's Stoltz. The problem is that Fox's costume in the 1985 scenes was different to Stoltz's. On his feet, Fox wore white Nikes with a red swoosh, but Stoltz wore black and white Converse shoes. Since the person leaping into the DeLorean wears Nikes, it isn't Stoltz. That doesn't mean it was Fox. Even simple things like this can be classed as stunts, and the amount of time the crew had to film with Fox was extremely limited, so any shots where you can't see his face could easily have been a stunt double.
The second shot is a very quick shot in Lou's Diner when George is talking to Lorraine. People claim you can see Eric in the background, instead of Michael. I can't make out who it is, but he seems to be chatting to some other guys - which I don't think Marty would do. Also, I'm sure the same person can be seen in the background throughout the scene, including when Biff's gang enter. By this time, Marty needs to be round the other side of the diner to trip Biff up. For this reason, I think it's just an extra.
The third shot is of Marty's arm as he punches Biff in the same scene. There's no reason why they would film this again, so I reckon it's probably Eric.