It's 18 years since Brad Pitt burst on to our screens in his scene-stealing cameo in Thelma & Louise. As the hustler with the blond locks and tanned six-pack who gave Geena Davis her first orgasm, he was the perfect Hollywood specimen. Even then, his crowning as People magazine's Sexiest Man Alive – a title he took in 1995 and again five years later – seemed a mere formality. Yet as he enters the room today, an opulent Berlin hotel suite near to where he's filming Quentin Tarantino's new Second World War film Inglourious Basterds, he appears a world away from his humble "himbo" beginnings.
Now 45, Pitt has aged remarkably well. He has recently been filming the role of Lieutenant Aldo Raine, the leader of a group of cut-throat Jewish-American soldiers called The Basterds. His hair, swept to one side and shaved at the back with military precision, is a darker, dirtier blond than it once was. His face looks angular and thin, while his physique seems slight, certainly nowhere near the musclebound bulk he put on when he played Achilles in 2004's Troy. Maybe it's that Pitt is now so frequently photographed with partner Angelina Jolie, he seems incomplete without her.
Ageing, of course, is the main topic of the day. Pitt is currently promoting The Curious Case of Benjamin Button, his third collaboration with director David Fincher, after Se7en (1995) and Fight Club (1999). Loosely based on an F Scott Fitzgerald short story, the film casts Pitt in the title role: a man who is born old and ages in reverse, growing younger every day. Technically remarkable, the film sees Pitt's features aged accordingly and grafted on to the face of a body double.
"I'm working on a way to possibly see if we can film all movies in the future from your own armchair at home!" he grins.
Not that he could be accused of phoning in his performance. With Benjamin narrating his own "unusual circumstances", as he puts it, from his birth in New Orleans at the end of the First World War, Pitt has the difficult task of playing a man who's an observer in his own life. It's a far cry from, say, his suave crook Rusty Ryan in Ocean's Eleven (2001) and its two sequels, Fincher calls it "the stillest performance" in the actor's career. Pitt nods at the thought of this. "Some parts are more gregarious, some are about internalising the moments," he says. "This film, the process is more about watching the other actors and responding to what they give across the page."
Certainly, critics and peers are in agreement that it's up there with his best work – with Pitt gaining Best Actor nominations at the Golden Globes, Baftas and Oscars. His only previous nod at the Oscars was in the supporting category for his asylum patient in Terry Gilliam's Twelve Monkeys (1995), though he plays down this current attention. "To me, the biggest battle is making a film you're proud of," he says. "That's the win for me. That's what I feel we've done. Awards are really nice. It's a fickle business. If your number comes up, it's fantastic. Of course the Oscars are the highest honour, and we're more than happy to be there. If not, we still walk away with a great film, and really I think that's the healthiest way to play it."
While the film is primarily a romance between Benjamin and Daisy (Cate Blanchett), the young ballerina he meets in his early years who grows up as he gets younger, it could just as easily be seen as a love-letter to Pitt himself. Fincher toys with his movie-star iconography, notably in a scene when Pitt is playing the middle-aged Benjamin without make-up. Riding a motorbike, looking like Steve McQueen in a leather jacket and shades, it's as if we're meant to acknowledge his place in Hollywood history. "I think it's probably a ridiculous notion to cast Brad Pitt as the Everyman," smiles Fincher. "But there's a real benefit to having somebody that you can't walk 50ft in the civilised world and not see a photograph of. People are so familiar with his face, you can work with that."
With the film a meditation on life, death, ageing and beauty, Pitt admits that it was playing the wizened Benjamin that held the most fascination. "I've done the younger version so it wasn't as interesting as what the future might hold," he says. Heavily involved in testing the digital effects during pre-production, Pitt "got a little bit of say" in what his character looked like with wrinkles. "I can't say time will be as kind as I was to myself." He even got to show his ever-growing brood of children just what he'd look like in years to come. "When I was in the full make-up, we had the kids come to the set, and I tried to prepare them for the moment. Their mommy tried to explain to them that 'that's daddy'. But it didn't phase them. They didn't even comment on it."
Pitt now has six children with Jolie – three adopted and three that she has given birth to, most recently twins, Knox Leon, and Vivienne Marcheline, who arrived in the world last July. "I don't think I would've been right for this film without being a father," says Pitt. "It changes everything. It puts a greater value on my time and how I spend my time, and what I do with that time. All those things become immensely important." As the film testifies, it's a great responsibility. "As a parent, your job is to show them around and prepare them for going out on their own. It's a really high line. And they drive you crazy!"
By all accounts, Pitt's own family life was relatively stable. Born in Oklahoma, he was raised with a younger brother and sister in Springfield, Missouri. His father was an executive at a transport company while his mother was a schoolteacher, and they brought up their children as Baptist. Ironically, given how much attention he commands in the press, he trained as a journalist at Missouri State University, before abandoning his degree when he decided to head for Hollywood and ply his trade as an actor. He did odd jobs – everything from delivering fridges to ferrying strip-o-gram girls to parties – before the roles starting coming in.
After Thelma & Louise put him on the map, it didn't take long before he moved from bit-part player to leading man in films like Legends of the Fall and Interview with the Vampire. By 1995, he was a tabloid favourite, after he began dating Se7en co-star Gwyneth Paltrow, to whom he became engaged. He'd already been in relationships with Robin Givens, whom he met when filming an episode of sitcom Head of the Class, and Juliette Lewis, his co-star of 1993's Kalifornia. But in many ways a dry run for the Brangelina hysteria, Pitt and Paltrow, in the words of one critic, "were the only true heartthrobs of the mid-Nineties".
The same, perhaps, wasn't true of his five-year marriage to Friends star Jennifer Aniston, which came to an end in 2005, just after Pitt had met Jolie on the set of marital comedy Mr and Mrs Smith (2005). Once again, the press had a field day as Pitt and Jolie hooked up. Aniston recently branded what her rival did as "very uncool" – though it's not difficult to see that Pitt has a problem with commitment. More than once, he has left projects at the 11th hour. In 2002, he bailed on Darren Aronofsky's The Fountain, despite having grown a ZZ Top-like beard for his role as a time-traveller. His reason? To spend more time with Aniston.
He also recently pulled out of State of Play, Kevin Macdonald's film version of the BBC political thriller series. What it does show is just how cautiously Pitt is approaching his career. Though careful enough to satisfy the studio bean-counters with sporadic blockbusters, he has mixed them up with a series of more esoteric projects. From his frantic tourist in Babel (2006) to the bubblehead gym-bunny in the Coen Brothers' Burn After Reading (2008), Pitt's recent choices have been more daring than his days in dumb films like The Devil's Own (1997) and Meet Joe Black (1998). Furthermore, as he steers away from trading on his looks, he's emerging as a far more multi-faceted actor than old-school A-list stars like Tom Cruise and Bruce Willis.
Indeed, in between childminding and charity projects with Jolie, it seems that Pitt's main objective is to collaborate with cinematic legends. He recently worked with the Terrence Malick on his 1950s Midwest-set story The Tree of Life, replacing the late Heath Ledger.
"As you see, I do appreciate an auteur," he says. "It's a director's medium, I say, first and foremost. They're the storytellers. So you want to put yourself up with original storytellers. Guys that are obsessed with their craft." Yet it's his role for Tarantino that will cause the most consternation this year. "The character is... I think his springboard is The Dirty Dozen," he explains. "He's of the past. All I can tell you is that he is absolutely outrageous!" Now that's tantalising.
'The Curious Case of Benjamin Button' opens tomorrow
The Independent Film Club
Next Wednesday sees the launch of The Independent Film Club. Each week you'll have the chance to air your opinions on a classic or current film, starting with 'The Curious Case of Benjamin Button'. Is it worthy of all those Oscar nominations? Watch the film this weekend, then post your thoughts online at www.independent.co.uk/filmclub . We'll print the best in Wednesday's paper.