Mask actor that began filming as Marty in Back to the Future before being dumped

It seems fitting that Back to the Future - the classic '80s time travel adventure about serendipitous intervention, chance meetings, and changing fates - might have been very different. In the film, now back in cinemas to mark its 40th anniversary, Marty McFly (Michael J Fox) travels from 1985 to 1955 in a DeLorean time machine and - Great Scott! - disrupts the moment his parents were destined to meet and fall in love, which threatens to erase Marty from history altogether.

With the help of his scientist pal Dr Emmett "Doc" Brown (Christopher Lloyd), Marty must get his teenage parents to fall for each other - then return to 1985 by harnessing the power of a history-making lightning storm.

But had the timelines aligned in any other way, if just one of the decisions behind the film had played out differently, Back to the Future might have featured a time-travelling fridge powered by Coca-Cola and chimpanzee sidekick - all details in early drafts of the script.

It could also have starred a completely different actor in the lead role.

Eric Stoltz was cast and filmed for six weeks before director Robert Zemeckis fired him and turned back time - in movie production terms - to film it all again with Michael J Fox.

If those timelines had aligned differently, it doesn't take Doc's four-dimensional thinking to figure out that Back to the Future wouldn't have become what it ultimately did: a bolt of clock tower-striking lightning in a bottle.

It's a film that not only captures but defines a particular kind of '80s movie magic.

If the secret to time travel was the Doc's "flux capacitor", the secret to Back to the Future's overall brilliance is a combination of elements that became instant cultural touchstones.

It's the pairing of Fox and Lloyd as Marty and the Doc. The DeLorean. The chocking chords of Huey Lewis's theme song, The Power of Love.

The endlessly quotable lines ("Roads? Where we're going, we don't need roads!"). It's town bully Biff Tannen (Thomas F Wilson) getting his comeuppance via a truckful of manure. And the nostalgia-stirring score by composer Alan Silvestri, a rare film score - along with Star Wars, Superman, and Indiana Jones - that really can transport you back in time. Right back to childhood.

"Pull out one of those elements and we wouldn't be having this conversation 40 years later," says Bob Gale, who created and wrote the film with Zemeckis.

Back to the Future's huge success led to another two films, creating one of the finest trilogies - 1.21 gigawatts of the purest form of Hollywood entertainment. Each of the three films is released on a 40th anniversary 4K steelbook collector's edition next week.

The story of Back to the Future begins just as the film ends: with a destiny-changing storm. It was 1980 and Gale visited his parents after their basement had flooded. His father had salvaged some items, including a 1940 school yearbook. Leafing through the book, Gale was surprised to see his own father as a teenager.

"I discovered my dad was president of his graduating class," says Gale. "That was completely unknown to me. I was looking at this picture of dad looking very straight and proper, and I wondered if we'd have been friends if I'd been in his class. I ultimately said, 'No, I probably wouldn't!' It was also the moment I said, 'That's a really good idea for a movie - that a kid could end up in high school with his dad.'"

Indeed, the eventual story is as much about Marty's dorky dad, George McFly (Crispin Glover). It's Marty who travels to the past and inadvertently invents skateboarding and rock'n'roll. But it's George whose future is changed when he stands up to Biff and follows Marty's advice. "If you put your mind to it, you can accomplish anything," Marty tells him in 1955 - a piece of advice his dad repeats back to him 30 years later.

Back to the Future didn't immediately hit 88mph. It stalled. An early script was rejected all over Hollywood. "Studios would say, 'Time travel movies don't make money!'" says Gale.

Disney baulked for another reason: because the 1955 version of Marty's mother, Lorraine (Lea Thompson) gets the hots for him, not realising Marty is her future son.

"Disney rejected it on the basis that it was about incest!" says Gale, laughing.

The breakthrough came after Zemeckis scored a blockbuster hit with Romancing the Stone, the Michael Douglas and Kathleen Turner adventure film.

Zemeckis was suddenly an in-demand director. The time was right for Back to the Future. It was at this point that they decided to put the time machine in a DeLorean. As the brilliantly manic Doc says in the film, "The way I see it, if you're gonna build a time machine into a car, why not do it with some style?"

It's hard to believe that Back to the Future would have raced into the imaginations of cinemagoers in the way it did without the DeLorean. Or that anyone would remember DeLorean cars and their gull-wing doors if one hadn't appeared in Back to the Future.

Forty years on, the film and car are indelibly tied to each other.

With Back to the Future greenlit, Steven Spielberg - a staunch supporter of the project from the beginning - served as executive producer. Though the story belonged to Gale and Zemeckis, it's also pure Spielberg fodder - part of a run of '80s childhood-shaping hits directed or produced by Spielberg in which fantastical events come to all-American suburbs. E.T., Poltergeist, Gremlins, The Goonies.

When it came to casting the role of Marty McFly, Michael J. Fox was the first choice.

But Fox was unavailable due to commitments on his sitcom, Family Ties. Universal Studios boss Sid Sheinberg pushed instead for Eric Stoltz, whom Sheinberg saw as Hollywood's brightest new star.

"We pushed back," remembers Gale. "But in a moment of exasperation and super self-confidence, Sheinberg said, 'I'm so certain that Eric Stoltz will be great as Marty McFly that if he's not, come back to me with a different actor and you can start all over again.'

"Of course, Sid never thought we were gonna take him up on that..."

Stoltz approached it with method acting.

He insisted everyone call him Marty and distanced himself from the rest of the cast, creating an uneasy atmosphere. Stoltz was also playing the part too seriously.

Six weeks into shooting, Zemeckis knew there was a problem with the performance. "Bob described it as there being a 'black hole in the middle of the movie,'" recalls Gale. "Eric was just not giving us the comedy the character needed."

They decided to replace Stoltz and re-shoot his scenes. Stoltz took the news well but later described his firing as "devastating". This time around, the producer of Family Ties gave permission for Michael J Fox to make the film - on the condition the actor prioritised Family Ties and Back to the Future was scheduled around his sitcom commitments.

Consequently, Fox would shoot Family Ties in the day and Back to the Future at night. "We had a very tired Michael J Fox!" says Gale.

He adds: "It's funny how much the film industry has changed. Today, there is no actor's attorney, no studio attorney, no business affairs department, and no insurance company that would allow an actor to make a TV show and movie at the same time!"

Fox was, of course, perfect for Marty - the embodiment of streetwise '80s cool, dressed in slick '50s style - and about as likeable as any actor in Hollywood history.

The peak Marty McFly moment comes when - after successfully getting his parents together at the school dance and averting a space-time continuum disaster - he performs a rendition of Chuck Berry's Johnny B. Goode.

But even that scene - one of the greatest musical scenes ever put on film - almost didn't make it. Gale recalls that Zemeckis suggested cutting the Johnny B Goode sequence to make the film tighter. "Our editor, Arthur Schmidt, said, 'Bob this is a good scene! At least take it to preview screenings and see what the audience thinks of it. They might really like it.' And they did... They more than liked it."

In the film, Marty's playing inspires Chuck Berry when Chuck's cousin Marvin puts in a quick call ("You know that new sound you've been looking for? Listen to this!").

But it inspired numerous musicians for real - not least of all Coldplay's Chris Martin, who credited the Johnny B Goode sequence for making him want to be in a band.

When Back to the Future was released on July 3, 1985, it became the highest grossing film of 1985 and remains a cultural phenomenon to this day.

Part of the fun is how dense the film is with pop culture references and echoes across the two time periods. Forty years later, there are still little details to discover.

In the final scene, the Doc returns to whisk Marty off to the future.

"It's your kids, Marty! Something's got to be done about your kids!" It wasn't intended as a set up for a sequel - just a great punchline.

But when the studio demanded a sequel, Gale and Zemeckis realised they'd written themselves into a corner - they had to come up with a story to send Marty and the Doc to a then futuristic-sounding 2015.

Part II arrived in 1989 and Part III, which finished the story in the Old West, followed in 1990.

Back to the Future remains mercifully untouched by further sequels, remakes, or reboots. There was talk of a Part IV in the 1990s, but Spielberg prevented the studio from making one without Gale and Zemeckis's approval. "He said to us, 'If you don't want any more Back to the Future, there won't be more Back to the Future,'" says Gale.

Instead, they created Back to the Future: The Musical, which opened in 2020 and is still playing in the West End - a spectacle that's like experiencing the first film all over again.

Back to the Future might be '80s movie magic but it's also timeless.

• The 40th Anniversary 4K Steelbook Collector's Editions of all three films are available to buy from November 10 at Amazon, HMV and Zavvi