The screenwriter Eric Roth, breezily tapping the time-spanning spirit of his latest cinematic opus, The Curious Case of Benjamin Button, asks: “How far back do you want to go? I can take you back 80 years if you like.” The movie, a Brad Pitt blockbuster that doubles as a metaphysical head-tripper, is both a technical triumph and a serious awards season contender (13 Oscar nominations can't be wrong). Moreover, it boasts the kind of epic production history, complete with births, deaths, break-ups and make-ups, that has already earned it a place alongside Gone With the Wind, Cleopatra and Titanic, in that rare Hollywood pantheon of demented and truly inspired passion projects.

“In 1922 F.Scott Fitzgerald had a baby girl,” continues the 63-year-old Oscar-winner Roth (Forrest Gump). “And when she was three months old he wrote a short story, The Curious Case of Benjamin Button.” The tale, about a man who was born at 70 and slowly aged backwards towards infancy,reflected Fitzgerald's newly altered views on mortality, Roth says, adding: “But it was very broad and whimsical.” However, the basic idea would eventually evolve into the story of an 86-year-old man, Benjamin (Pitt), who is born as a wizened homunculus in a New Orleans hospital in 1918, and who nonetheless gets younger and stronger as the decades pass, while those around him are ravaged by old age. His one chance at love, with a young dancer called Daisy (Cate Blanchett), is realised only with a tragic, fleeting bliss.

The story was picked up, Roth says, by the Hollywood heavyweight producer Ray Stark (The Night of the Iguana; Funny Girl) in the late 1950s. Stark sat on it until the early 1980s, when “various writers” started attempting adaptations. “The most predominant writer attached was Robin Swicord (Little Women) and she was probably on it for 12 years, on and off,” Roth says.

It was during this period that David Fincher, a 30-year-old commercials director, read Swicord's script for the first time. “It was 1992 and I knew that Steven Spielberg had been interested in it but had cast it off,” says Fincher, who subsequently went on to direct touchstone movies such as Seven and Fight Club (both with Pitt). “I read it, thought it was lovely, but didn't know how it could be done.” The mind-boggling technical challenge the film posed, he explains, was the credible depiction of an ancient head on a child's body.

By 2001, Fincher's friend and fellow tyro Spike Jonze (Being John Malkovich) began developing a Button adaptation with the screenwriter Jim Taylor (Sideways). “He said it was going to be a much more intimate story,” Fincher says. But when Paramount Pictures (which owned the rights) announced that it was passing the adaptation challenge to Roth, then boasting another Oscar nomination (for his Insider screenplay), Jonze dropped out. “I remember saying to Spike, ‘Y'know, Eric Roth is pretty f***ing talented. Maybe you should wait to read what he does before making your mind up'.”

Roth, meanwhile, was a man burdened. His mother, who had cancer, died three months after he had started writing his Benjamin Button. His father died less than a year later. Unsurprisingly, he says, and considering the subject matter (mortality and the inevitability of death), the ghosts of his parents made it into the script. “There was a lot of my own personal look at death in there,” he says. “ I used a lot of things that were said to my mom and dad at the hospital in the dialogue we used .”

Fincher, he says, was blown away by Roth's finished script, which by 2002 was running to more than 200 pages. “I said, ‘I don't know if the studio will pay for a four- hour movie, but I'd love to be involved!'” Some Hollywood careerist manoeuvring ensued until Fincher found himself standing before the Paramount studio chief Sherry Lansing, explaining why his version of Benjamin Button was going to cost $150 million (£108 million).

“There was so much special effects in there that had never been done before,” he says, referring to the movie's prototypical “head replacement” effects technique that would allow Fincher to graft any adult actor's above-the-neck performance on to the body of a smaller actor; they eventually used “little people” and not children. “Initially the studio balked at the price,” he adds. “But over the three years of development, as more expensive movies, such as Spider-Man 3, started coming out, $150 million didn't sound too insane.”

Pitt, Fincher's friend and favourite leading man, came on board, but with one proviso. “Brad was interested in playing the part only if he could play the character through the totality of his life,” Fincher says. Thus the effectiveness of the as yet untested head replacement technique was central to a film that would feature the familiar visage of Pitt in nearly every scene, and spanning 86 years of screen time. The head replacing itself, Fincher explains, involved sitting Pitt in front of five cameras and filming his facial performance in close-up, before digitally mapping that performance on to an actor's body. “Brad took to it like a duck to water,” he says. “We joked that he'd never have to leave his home in France again. Just get a head replacement studio in the garden.”

The filming process was duly epic and spanned 150 days in Louisiana, Montreal, Los Angeles, the Virgin Islands and Cambodia - Benjamin, towards the end/beginning of his life, goes on a soul-searching world tour. However, in 2007, after eight months of post-production, manipulating more than 200 hours of film, Fincher finally hit the wall. It wasn't working. “It was a disaster,” he says, recalling the endless days of post-production hell. “I'd look at the shots and go, ‘Oh, my, God! I am going to get sued and spend the rest of my life in debtors' prison!'” Eventually, though, somehow a small ten-second shot of the octogenarian Benjamin, with the wrinkled head of Pitt on the diminutive body of the actor Peter Donald Badalamenti, bashing a fork against the table in the movie's retirement home, came together on screen. “One day, Eric Barba, the visual effects superviser said, ‘I think we got one!' And that was it. Benjamin had become a character. And I don't think there's a better shot in the entire movie,” he says.

And true, the finished effects are impeccable and often eerily compelling. While it's a testament to Pitt's facial performance that even the most inquisitive viewer, unable to spot the joins, will quickly slip into the fantasy of the shot.

Yet the shots aren't everything, Fincher admits, and neither is the awards season buzz that's swirling around his finished movie (“Though that would just be the icing on the cake!” he confesses). No, he says, The Curious Case of Benjamin Button is special not because of its monumental back story but because of the depth of feelings that inform it. Fincher's father, Jack, died just before the movie was given the greeen light by Paramount.

“The poignancy of the film was informed by losing my dad,” he says. “He had been sick for over a year, but one night he got a blood clot. I was there the moment the doctor told him, ‘You have five minutes!' And there was a look on his face that said, ‘Oh my God, this is the last time that I'll see you'. I tried to transfer that to the movie.”

Similarly, Roth concludes that the film is at its best when it gets you thinking and puts you in a better place. “It reminds you that you have only so many days and that you should try to make the most of them,” he says. And that, from a movie that began 87 years ago and spent a lifetime coming to the screen, is ironic, paradoxical and, much like the film itself, sweetly moving.

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