https://backtothefuture.fandom.com/wiki/Eric_Stoltz#:~:text=Eric%20Cameron%20Stoltz%20(born%20September,began%20on%20November%2026%2C%201984.

https://www.digitalspy.com/movies/a655466/why-original-marty-mcfly-eric-stoltz-was-fired-from-back-to-the-future/

https://www.youtube.com/watch?v=0JGNkfdFy-8

https://www.youtube.com/watch?v=_wudNasQbv0

@Knixon said:

I was leafing through some back-issues of People magazine, discarded from the local small-town library, and I came across a reminder that Eric Stoltz was originally cast in the role of "Marty" for "Back To The Future."

While he was acknowledged to be doing a fine job, he was playing the role basically straight. (And many who knew him claimed that Stoltz couldn't be funny to save his life.)

The producers etc had a feeling the movie needed to be played more as a comedy, and even though filming was almost if not entirely completed, they took the chance and replaced Stoltz with Michael J Fox and pretty much re-filmed the movie.

And it sure paid off! Especially with the sequels etc.

But can you imagine BTTF coming out, and going to see it, and it's NOT FUNNY???

Well, I can imagine it not being as funny as with Michael J. Fox, but it would still be a bit funny.

It's too bad that Bob Gale doesn't want to release the 6 weeks of footage they filmed with Stoltz. The reason he gives:

“Eric’s a working actor [and] a working director,” Gale says. “The idea of shining a light on that and having everybody snipe at him on the internet, which is what they would do. ‘Oh, my God. He sucks.’ Whatever nasty things people would say, we don’t want to do that to him. He doesn’t deserve that. He came to work. He did the best that he could. We made a mistake and we cast the wrong guy. We were able to rectify that, but we don’t want Eric Stoltz to go down in history, his biggest claim to fame, like Pete Best and the Beatles, he’s the guy who wasn’t in Back to the Future. Eric’s done some really good movies, both in front of and behind the camera. And we do this out of respect for him and to not denigrate him.”

I think you underestimate the possibility that with a non-funny actor, and considering how everything else would be playing off that - Doc Brown's over-acting would have changed too, most likely - the whole thing becomes flat. I mean, even "You made a time machine out of a Delorean" can be made flat and unfunny if someone else says it that way.

Well, without any footage - except for the couple of seconds released - I can't say how funny it would be.

Michael J. Fox was perfect for this role and when there is a reboot or sequel, I doubt that an actor or actress can be found to be as funny.

Hollyweird people need to learn to leave things alone.