After his film became a word of mouth hit at the Sundance festival in January, British director Bart Layton was flown to Hollywood for "a mad tour" of the big studios. When he got there, though, he realised no one quite knew what to make of his work. "People are desperate to find categories," he says. "They say, is it documentary? Is it fiction? Americans refer to everything that isn't documentary as 'narrative', but I would absolutely argue that this is a narrative film."

"This" is a documentary called The Imposter, the story of Frédéric Bourdin, a mixed-race Parisian who, in the late-90s, adopted the identity of Nicholas Barclay, a boy seven years his junior. Bourdin's age – 23 – and appearance – stubbled and swarthy – didn't stop the Barclay family from claiming him as their missing kin. Neither did the fact that Nicholas, now with a French accent, was found in Madrid, Spain, despite having disappeared on his way home from basketball practice in San Antonio, Texas.

Layton came across the story while flicking through a magazine at a friend's house. The piece didn't go into much detail, but it did reveal that Bourdin, also known as The Chameleon, had had a career impersonating destitute children, travelling all over Europe claiming to be an orphan or a damaged kid, with what seemed to be the express goal of being taken into care by social services.

"When I went home and did a bit of research," says Layton. "I found more stories, one in the Guardian actually, that went into huge detail, not just into his back story but also an incident in which he successfully stole the identity of a real missing kid. If you'd read it as a work of fiction you'd think it was far-fetched, so I was immediately intrigued. I had two questions straight off: one, what kind of a person would do that? And two, what kind of a family might fall victim to it?"

The starting point was to get Bourdin on camera, and, after persuading the oddly public Bourdin to come to London, Layton conducted two days of interviews. "When you meet Bourdin," says Layton, "you have quite a complicated series of reactions, because it's not straightforward. On one level, he certainly invites sympathy. You do feel he needs looking after. He can be charming and disarming, yet at the same time what he's telling you can be quite repellent. Which got me wondering whether the story was going to be specifically about him. Or was he the entry point for a bigger story?"

Bart Layton - director of The Imposter. Photo: Alex Schneideman

The latter, it turned out – and the masterstroke of Layton's film – is that Bourdin becomes something of a victim himself, as the film shifts gears to reveal some hair-raising facts about the Barclays. "The thing about Bourdin," says Layton, "is that when he starts telling you his story, very soon you find yourself thinking, 'Hmm, maybe there is some logic to this …' Even though it's a very twisted logic. You hear him tell his story of having had his own childhood taken from him – so wasn't he justified in stealing one? They were a family without a kid, he was a kid without a family …"

"I started to realise that I was falling for the con, and that this master manipulator was doing to me what he'd done to everyone he'd come into contact with. That's when I thought maybe there was a different kind of film to made here. One where you're not looking at these characters and this chain of events through a microscope – you're inside the story."

So the film starts with Bourdin making the phone call that took him to Texas. It's when the Barclays appear, roughly 20 minutes in, that a dark, already disturbing film becomes scarily gothic. You wonder, in fact, why the family would even want to co-operate.

"What kind of person could not know their own flesh and blood?"

"They're people who have seen a lot, and so they're not perhaps precious in the way you might imagine," says Layton, "even though there's a missing child at the heart of all this. I think they knew that Frédéric would be doing his 'thing', and I explained to them that, strangely, I thought it was important for the audience to see that. Most people would come into a film like this and think 'what kind of person could not know their own flesh and blood, if they've only been missing for three years and returned with brown hair instead of blond, brown eyes instead of blue, and a thick French accent?' It is laughable. It is absurd. But when you allow Bourdin to do what he does, you get a very different experience."

As a result, The Imposter feels more like a fiction film than a doc, which is what confused those Hollywood executives. Did Layton ever look at fiction films for inspiration? "Constantly. People have made flattering comparisons to Errol Morris's The Thin Blue Line, which is an incredibly important film, and without that, the likes of Touching The Void, Man On Wire and this film probably wouldn't have been made in the same way. But I was thinking more of fiction films. I thought about The Usual Suspects a lot, being a film with an unreliable narrator at the heart of it."

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Everything you see in The Imposter, however, is most definitely real. "This absolutely happened in the real world," insists Layton, "and there's nothing fake, even though people have questioned whether the interviewees are actors. They're absolutely not. But it also has a foot in the Coen brothers' world. A lot of them are characters in their own strange movie. Particularly Bourdin."

Indeed. As Layton prepares to make his next movie, "an existential heist movie" starring James Franco, Frédéric Bourdin is already getting accustomed to a certain bizarre stardom of his own. If you're inclined, you can follow him on Twitter on @Francparler.

"I'm not only a movie," he tweeted recently. "I'm real."

The Imposter is at selected cinemas from Friday