The Imposter, Bart Layton, 95 mins (15) F for Fake, Orson Welles, 85 mins (PG)

The latest dazzling docu-drama reveals less than Orson Welles's late masterpiece on fakery

We've been seeing a lot of documentaries recently that focus on the strange but true – stories that you just couldn't make up. The latest is The Imposter, about an outrageous act of deception. Mixing interviews and dramatic re-enactments, Bart Layton's film begins with the disappearance in 1994 of a 13-year-old boy, Nicholas Barclay, from his home in Texas. Three years later, he miraculously turned up in, of all places, Linares in Spain, and was soon restored to his loving family.

Except that he wasn't. The boy who claimed to be Nicholas was actually one Frédéric Bourdin, and he wasn't even a boy. He was a 23-year-old Frenchman, who would turn out to be a serial impersonator of lost children: a Raffles of identity theft.

The story is hair-raising: Bourdin claims he learned about Nicholas's existence by making a phone call from a Linares police station in the guise of a Spanish detective. Deciding that an American teenager was as good a role to adopt as any, he persuaded the boy's family that he was Nicholas – even though he looked and sounded nothing like him. The film's most disconcerting revelation is how easily people are fooled when they want to be. Nicholas's family desperately needed to believe Bourdin was the genuine article, but truly alarming is the FBI agent who let the lie get the better of her, even while staring the evidence in the face.

Bart Layton has a great story here, and larger-than-life characters: including the improbably named Charlie Parker, an astute private investigator who appears here both as himself and played by an actor. The film's structure is confusing, deliberately, it seems: Bourdin, as he poses as Nicholas, is himself played by one Adam O'Brian, but also appears as himself today, telling his story straight to camera.

Bourdin is presented as, in effect, the narrator/ringmaster of The Imposter, and there's something immensely troubling in the way Layton has him regale us with his confession – a testimony that, while seemingly authentic, inevitably comes across as suspect, for how we can believe a self-confessed inveterate liar? It's quite shocking how pleased Bourdin seems with himself; just listening to him, it's hard not to feel uncomfortably complicit with his crimes. His deception of Nicholas's family (involving wild fabulations about abduction and abuse by the military) was a shockingly callous act, yet Bourdin seems still delighted and amazed that he pulled it off. Nicholas's mother describes her lost boy as "a little pixie"; the intensely creepy Bourdin is more like a malign goblin, pulling a fairy-tale changeling routine. That, in a further twist, Bourdin comes up with his own amazing theory about Nicholas's disappearance really comes across as damnable cheek.

With its elegantly staged reconstructions, film noir atmospherics and ominous music (shades of Errol Morris's genre-blurring The Thin Blue Line), The Imposter tells us that truth and fiction are closer than we like to think – but to what effect? A nightmarish true story is reduced to the status of bizarre anecdote. Layton seems less interested in the head-spinning complexity of life than in the stylistic possibilities of making truth as exotic as the most outlandish fiction. Bourdin's story doesn't need dressing up; it's already as outré as they come.

This is not the only recent documentary that makes you wonder whether you're being told the truth at all, or sold an out-and-out false bill of goods. The most questionable was Catfish, another tale of imposture –about which the jury, as far as I know, is still out. I'm all for documentaries, or quasi-documentaries, playing fast and loose, but I worry that uncertainty fatigue is making us excessively suspicious about facts presented on screen. This doesn't make for enlightened scepticism, but something akin to conspiracy-theory paranoia (I know people convinced, erroneously, that the recent music doc Searching For Sugar Man was a complete hoax).

Perhaps Orson Welles is to blame. His magisterial 1973 film F for Fake, now reissued, created its own genre: documentary as shaggy-dog story. This brisk, witty study of deceit spins a dizzying set of speculations around art forger Elmyr de Hory, Clifford Irving, creator of the phoney Howard Hughes memoirs, Picasso's infatuation with a femme fatale named Oja Kodar, and Welles's own status as magus and/or charlatan.

F For Fake is fleet of thought, and wonderfully entertaining, living up to its theme by pulling the wool over our eyes shamelessly. Welles reveals cinema itself (documentary included) as an outrageous scam. While celebrating the art of fraud, it warns against believing magicianly liars, either in movies or in life.

There are many different ways for documentary to play with reality, and give it a quasi-fictional twist. It made perfect sense for Man on Wire, about high-wire walker Philippe Petit, to stage his twin towers feat as a Seventies caper movie (see Nicholas Barber, below): it helped you understand the courage and ingenuity at stake. And it made sense for last year's remarkable Dreams of a Life to dramatise moments from the life of a woman found dead, so as to amplify the essential unknowability of an elusive subject.

The Imposter doesn't reveal layers of complexity; it just embellishes the facts with glossy stylistics. It doesn't illuminate Bourdin's crimes or psyche, but only revels in his flamboyant mythomania. I'm feeling burnt out by such wild-side documentaries. Perhaps it's time to say to docu-makers: just give us the facts, nothing fancy. Either that, or be prepared to really mess with them – ask yourself, what would Orson Welles have done?

Critic's Choice

Dividing critics is Sarah Polley's infidelity drama Take This Waltz. Take it with a pinch of salt, but see it for bravura direction and top performances by Michelle Williams and Seth Rogen. Sandrine Kiberlain excels as a woman in a corner in Yves Caumon's The Bird, a superb example of French psychodrama in a vein of reverie.