Bob Gale, co-writer/producer

Bob Zemeckis and I had written three movies together, and we had always wanted to do a time-travel story. We'd just never figured out how. What turned the light on for me was coming across my dad's old high-school yearbook and thinking, "Would we have been friends if we'd been at school together?" All of us have that revelation when we understand that our parents were young once, too. That's a big moment. Then there is the message that we all have control over our destinies. I thought we could dramatise those two things.

It took us a while to work out how Marty McFly would travel through time, however. We knew it had to be an accident. One of the conventions of time-travel movies is that it can't turn out well if the character is doing it for any kind of personal gain. But t here was no DeLorean originally. In the first two drafts it was a time-chamber apparatus. And the whole clocktower sequence at the end wasn't there: it was going to be the radiation from an explosion at a nuclear testing site that would power the time machine back to 1985. It was a bit too complicated. We also had George McFly, Marty's father, grow up to become a professional boxer after punching Biff, which didn't feel like the best solution to Marty's problems. Also, Doc Brown's inventions changed 1985 and made it much more futuristic when Marty finally got back. Without exception, our script readers had problems with that.

Although we'd done our first three pictures with Steven Spielberg, and he loved our Back to the Future script, we didn't want to make it with him at first. We thought we might become known as those guys who only get work because they're pals with Spielberg. What changed was that Bob had a big surprise hit directing Romancing the Stone: after that, he had proved himself, so we figured it would be fine to work with Steven [producing].

The craziest thing that happened was the cast change six weeks into production when Eric Stoltz was replaced by Michael J Fox. The humour just hadn't been coming through with Eric. The studio weren't happy exactly, but they'd seen the footage so they bit the bullet. Scheduling became quite complicated because Michael was doing his TV series Family Ties during the days. Then we'd get him from 6pm and work with him through until one or two in the morning. He was young, so he could go without much sleep. He was a real champ. We'd work round him: in the McFlys' dinner scene early in the film, everyone else gets two-shots, but Michael is seen in closeup because his shots had to be done separately.

Crispin Glover, who played George, was really something. "Nuts" is a good description. A lot of those mannerisms were actually his. He had to be reined in occasionally. The hardest thing was getting him to act like a completely normal person for the final scenes of the movie.

The first preview went brilliantly, though there was some nervousness when Einstein the dog disappeared during the time travelling – everyone thought we'd killed him. The studio head was so excited about the film he asked if we could bring the release forward from mid-August 1985 to early July. He'd give us whatever it would take to make that happen. Nine and a half weeks after we'd wrapped, it was in cinemas. We ruined post-production schedules for ever: everyone would be told, "Those Back to the Future guys did it fast; why can't you?"

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Dean Cundey, director of photography

Romancing the Stone had been a very muddy, arduous shoot, so Back to the Future was simple by comparison – most of it was shot on the lot at Universal, or in neighbourhoods in Pasadena. The most difficult part was the change in lead actor. I remember when we were about five weeks in, I had my suspicions that people weren't exactly pleased with the casting, and that some kind of change was in the air, but no one had said anything. We were shooting at night, and Bob Zemeckis called the crew together and said he had good news and bad news. The bad news was that we were changing our main cast member and that we'd have to reshoot everything we had done. We all looked at one another, and the general consensus was that this wasn't bad news.

That sounded like good news to us!

CRISPIN GLOVER, BACK TO THE FUTURE Photograph: Allstar/UNIVERSAL/Sportsphoto Ltd./Allstar

Working late was fine at the start of a week but when we got to Friday, shooting almost all night every night, it was pretty tiring, though it was countered by the fact that it was such a fun shoot. Michael had to work through his fatigue. He'd be very "up" during a take but then I'd notice him in his chair afterwards, desperately trying not to fall asleep. He was so engaging, though – he really had a great effect on set. Crispin Glover's casting I was dubious about at first. He was such an off-the-wall person that I didn't know if it would work. In fact, it made George more of a stand-out character

Once the film came out and was this huge success, the studio very quickly said, "We must have a sequel!" But because the second one was such a long script, and they couldn't find any way to shorten it, they decided to make it as two films. I'm surprised the second one wasn't marketed as "part two of three". A lot of people found it less rewarding because it didn't have the fun and lightness of the first one, but then the third one makes sense of the second – it all connects up. I think the first film resonates so well with audiences because of the element of wish-fulfilment at its heart. We've all said to ourselves, "I wish I could go back in time and change something" or, "If only I could do that over again."

Secret Cinema Presents Back to the Future runs until 31 August. Details: secretcinema.org/tickets

More on Secret Cinema's Back to the Future

• The night in pictures

• Peter Bradshaw reviews the show

• Behind the scenes

More on Back to the Future

• My favourite film: Back to the Future

• 25 years of Back to the Future

Archive interviews with the cast

• Michael J Fox

• Crispin Glover

• Christopher Lloyd