In every movie, there will be scenes of ad-libbing or throwing in details that the filmmakers hadn’t envisioned, and the “BttF” movies are no exception. Most of George McFly’s mannerisms are just Crispin Glover acting naturally or putting his spin on the character, such as his laugh at the episode of “Honeymooners”.
He had to try and act “naturally” for the scenes of his improved 1985 self. Huey Lewis actually improvised the line of “I’m afraid you’re just too darn loud.”
We all know Marty thanks to Michael J. Fox, but there was originally a different name attached, Eric Stoltz. The producers wanted Fox originally but went with Stoltz until Fox signed on. They filmed several scenes with the first Marty, including the cafeteria scene where Marty tangles with Biff.
Stoltz did his move wrong and almost broke Wilson's collarbone – Wilson planned to get revenge by really punching Stoltz at the end of the movie, but couldn't because Fox replaced him. In addition, Fox does a genuine spit take when he sees Lorraine smoking because he didn't know there was real alcohol in the bottle he held.
A Hostile Set
One of the reasons Fox was brought in so quickly was because Stoltz was something of a prima donna. Others thought he was too serious for the role, and so when Fox finished his latest project, Stoltz was out and Fox was in.
Neither Tom Wilson nor Crispon Glover got along well with Stoltz, according to them. In addition, Glover proved to be quite more demanding than his character — argumentative, and often battling with director Robert Zemeckis. But, thanks to Stoltz being fired, many of the actors were frightened of doing too much, thinking their jobs might be in danger.
They Thought it Would Flop
It took far too long for the first film to be made, and even then it wasn't smooth sailing. The script floated around Hollywood for years, a lot of executives wanted to meddle even after Universal Studios picked it up.
Stoltz was replaced with Fox, and even after Fox signed on, he was running on fumes filming both this and the show "Family Ties", leading to a few breakdowns and nightmares for Fox. It made his acting in Back to the Future a little more lifelike, but it was still a rough time.
No Sequel Intended
Every version of the movie these days will have the bright, colorful “To Be Continued” card at the end of the film as Doc Brown, Marty, and Jennifer take off.
This wasn't present in the first cut of the film, since the filmmakers had no intention of continuing the series, and the “cliffhanger” ending was made as a joke. Yet the film went on to break records and garner immense praise, and so a sequel appeared anyway. The “To Be Continued” card was hurriedly added to the home video release in order to advertise "Back to the Future Part II".