So who are you calling weird?

Louis Theroux: Body-builders, white supremacists, Sir Jimmy Savile - no one's too strange for Louis Theroux. But, behind the bravado, he says, lurks an old-fashioned British geek

When I arrive at the production offices for Weird Weekends, Louis Theroux is on the phone to Sir Jimmy Savile and I kill some time looking at an article he has written about female American body-builders, one of the subjects in his forthcoming series. Women with faces like Dolph Lundgren in lipstick and bodies like a baroque wardrobe in a thong, expose tendons and teeth alongside a lanky Englishman, who manages to give off an air of bashful fascination even in a photograph. And in their way, both the glossy pictures and the distant murmur of the telephone call are instructive. Theroux is never happier than when he's out there on the frayed margins of lifestyle choice, among the obsessives and single-issue zealots. But while quite a few reporters can force their way into the society of such believers, not that many can get out again with their relationships intact, particularly after transmission.

When I arrive at the production offices for Weird Weekends, Louis Theroux is on the phone to Sir Jimmy Savile and I kill some time looking at an article he has written about female American body-builders, one of the subjects in his forthcoming series. Women with faces like Dolph Lundgren in lipstick and bodies like a baroque wardrobe in a thong, expose tendons and teeth alongside a lanky Englishman, who manages to give off an air of bashful fascination even in a photograph. And in their way, both the glossy pictures and the distant murmur of the telephone call are instructive. Theroux is never happier than when he's out there on the frayed margins of lifestyle choice, among the obsessives and single-issue zealots. But while quite a few reporters can force their way into the society of such believers, not that many can get out again with their relationships intact, particularly after transmission.

You might have thought that Savile's call would be a difficult one - since Theroux's recent film about him included a bracing moment when the disc jockey was surprised by a question about suggestions of a sexual predilection which could easily get his house burnt down by a mob. But there are no hard feelings, apparently. Theroux had just had an invitation to stay which he'd had to cancel due to work commitments and when he explains this his face purses briefly in what looks like genuine regret.

The question of exactly what is genuine and what is not is a live one with Theroux. Is it faux naïvety that he deploys with such pinpoint accuracy, or the real thing? I put an early marker down for authenticity when we begin talking about his own involvement in the shows - the have-a-go element which, by his account, his producer is rather keener on than Theroux himself. A recent trip to Thailand, to look into that country's thriving export trade in connubial bliss, had tested Theroux's commitment to immersion journalism. "There's obvious limits to how much I can get involved there," he says, "especially since I'm in a relationship. I mean... I wasn't going to get married." He explains the last point very carefully, as if I might be thinking that his failure to go through with the nuptials was unprofessional. Surely that's the genuine article, I think, and it is only later it dawns on me that I may have been naïve myself in disregarding the possibility of sly deadpan.

That's not to say that there's no other evidence of a core of unforced bafflement at the heart of Theroux's act. The sentence he uses most frequently during our conversation is, "That's a tough one, isn't it?", uttered in tones of mildly anxious perplexity. I'd like to claim this as a testament to my powers of interrogation but in truth the questions are pretty bland - what it really marks is his willingness to worry away at the exact nature of what it is he does so effectively. He's also preserved an ability to be surprised by the world, despite his odyssey through its weirder regions. Talking about a film he has made about rappers - in which he tours the Deep South talking to stars and wannabes - he wonderingly reveals that one of his interviewees, a self-confessed pimp, had been unabashed about his day-job: "What makes it strange is that rather than minimizing his criminal dimension he seemed to want to exaggerate it even more." It doesn't seem to have registered that Rule Number One in the gangsta rap publicity handbook is to appear to be a man of firm convictions, preferably criminal - and this despite his filmed encounter with a rap stylist who gamely attempts to finesse Theroux's love of cats and red wine into an image of brooding ghetto menace.

This amphibian ability to move between the knowing and the naïve is, of course, a large part of Theroux's charm - or at least the reason that charm survives so well on screen. But he's helped in the field by another ambiguity, his dual nationality - his father is the American writer Paul Theroux and his mother a former BBC radio producer. When the couple separated an amiable custody battle for national identity began over Theroux and his brother: "Every summer we'd go to America and my dad would kind of try to de-programme us a little bit from our Britishness and make us be a bit more in the American rough-and-ready mode." The de-programming was not entirely successful - the question of whether he feels British now is not a tough one, because he answers "yes" without hesitation. But there is enough of America in him to make it the most promising territory for his excursions - "I do feel a little bit American as well. A certain kind of feeling of self-reliance and can-do and get-up-and-go. It sounds kind of corny but that is the heartbeat of the American psyche."

The fact that he was an Anglo-American co-production himself was instrumental in launching his career. He was working at Spy magazine in New York, a short-lived satirical magazine, when Michael Moore - one of the pioneers of documentary geekiness - was looking for an English correspondent to help satisfy the British financiers of his own series. Theroux applied and, two weeks later, was given his first appointment with mental instability. "They called on Saturday and said, 'Are you ready to leave on Monday to interview some millennialists?' I nearly didn't go."

That first encounter established Theroux's ability to make contact with the other side - and he's been steadily employed as a medium ever since - helped by the fact that he never makes it explicit where his loyalties lie. He never winks at the camera to highlight a joke or distance himself from the people he's with. This, he explains, is partly ethical restraint and partly sheer timidity. "I've got a lamentable craving to be liked by people, so when I've been with people I've been more worried that the person would somehow think I was making fun of them. The important thing is the bond of trust that exists between you and the person you're interviewing, so to violate that by looking at the camera... I could never understand why you would do that."

It's a principle that has resulted in some odd moments when eccentricity tips over into something more malign - Theroux isn't the kind of confrontational film-maker who will oppose racist opinions from the moral high ground. He's far more likely, as he does in a film on South Africa, to riffle through a white supremacist's record collection and try to pin down the point at which a passion for the music of Lionel Richie and The Temptations tips over into a conviction that there can never be common ground between black and white.

"In real life I'm so supine," he admits. "I'm embarrassed to say it but I would rather just listen to crap than create an awkward moment. In the context of the series I'm sort of on best behaviour, so I do." Working on the programmes has changed him in other ways, too. He claims to be naturally bashful - the sort of character who would shrink from amateur dramatics or public speaking - and yet the style of his reporting occasionally demands a hideous combination of both. This has got easier, he says, but not yet easy. "If you flex those muscles they get stronger, but having said that I still find some of that stuff quite petrifying." The programme comes before his peace of mind, though - in one of the more excruciating moments from the new series Theroux road-tests the dating empowerment techniques of one Ross Jeffries, an assignment which involves walking up to strange women and engaging them in conversation about their favourite constellations. "That was embarrassing," he recalls with a wince, "but that was totally self-inflicted. We shot a whole sequence with Ross and it was good but I was like, 'We're not going all the way here', so I said to the director, 'I have to try out the techniques'. I knew I had to do it to give the sequence a pay-off." The result is a classic Theroux moment - the sight of a man American enough to go for it and British enough to be appalled at himself for doing so. And whether this is a natural hybrid or a brilliant piece of artifice? Well, that's still a tough one, even after meeting him.

* The new series of 'Weird Weekends' starts on 25 September on BBC2 at 9pm