1984 – “Wait a minute, don't I know you from somewhere?”

For young Marty McFly, Zemeckis knew exactly which actor he wanted. Unfortunately, Michael J. Fox was currently filming TV series Family Ties, and his producer wouldn’t give him any time off to shoot a movie.

“Michael Fox is a great actor, he’s got a fabulous sense of comedy timing,” says Zemeckis. “He looks perfect for the part. He’s an everyman. He’s accessible as an actor. He’s likeable. He’s funny.”

In perhaps one of the most famous Hollywood casting legends, Zemeckis instead opted for Eric Stoltz, whose schedule was less restricted, and who had impressed Zemeckis in the movie Mask.

However, disaster struck when four weeks into BTTF’s shoot, Zemeckis realised that Stoltz was completely miscast. The actor wasn’t bad in any way, but crucially he wasn’t funny enough. Instead of playing up the comedy, he was taking a dramatic standpoint.

“Bob was cutting the movie as he shot it,” remembers Gale, “and he said to [producer] Neil Canton and me, after he screened 40 minutes of cut footage: ‘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.”

Despite the fact that recasting would add a massive $3 million to the film’s $14 million budget, Zemeckis was adamant that they should replace Stoltz. Happily, Stoltz had been having the same thoughts, and accepted that he wasn’t the right actor for the part.

Spielberg calls it “the hardest decision I’ve ever made”, but thanks to the time that had elapsed, Michael J. Fox was now free to become a part of the movie.

“Michael stepped in and it was just there,” recalls Christopher Lloyd, who was cast in the role of Doc Brown when original choice John Lithgow was unavailable.

“Michael Fox has a great sense of humour,” says Spielberg. “He’s got wonderful comic timing. His dry approach to Alex in Family Ties is what, for me, caused me to watch that show week to week.”

With Lea Thompson and Crispin Glover cast as Marty's mother and father, and Thomas F. Wilson in place as decade-spanning bully Biff, the movie was set to hit the restart button...

Next: January 1985 - "This is heavy"