'You can't in any way take advantage of or manipulate people who are vulnerable'

A documentary on autism has wowed the TV critics, says Simon Usborne

For his latest documentary, Louis Theroux was supposed to immerse us in the world of hardcore pornography. But nervous suits at the BBC pulled the film from its 9pm slot. "I was told it was because it was scheduled to follow Springwatch," Theroux explains. "I can sort of see the reasoning. You might get a older viewers who come out of blue tits and into something quite different."

The film will now be broadcast in June (at 10pm) and takes Theroux back to Los Angeles and a once-booming industry in crisis. It's classic Louis; he stands, saying little and with a range of looks (bemused, baffled, awkward) as eccentrics and people on the fringes of society demonstrate extraordinary behaviour, be they porn stars or Christine Hamilton.

His new documentaries observe the effects of neurological disorders on families. Extreme Love: Dementia airs this Thursday on BBC Two and follows a film about autism last week. There had been concern among parents of autistic children that the last thing they needed was the "Theroux treatment". His critics have accused the presenter of using a faux-naif, shambling persona to trick subjects into humiliating or degrading themselves. The morning after the the first broadcast, Theroux, 41, sips a cappuccino at a cafe near home in Harlesden, North-west London. He says he was nervous. "For the first time I was doing shows that have no real implied critique, and don't explore worlds some people would see as untoward. You can't in any way take advantage of or manipulate people who are vulnerable."

The reaction of critics as well as sufferers and their families has been generally positive. One of the few notes of disapproval came from an autism consultant, who left a comment at Talk about Autism, an online forum. She took issue with a sequence in which Theroux fails to engage Brian, who is severely autistic. "It showed a lack of respect for privacy," she wrote.

Theroux later bonds with Brian but he says the new focus was challenging. In the dementia film, he spends time with Gary, who forgets he has been married for 30 years. "I asked him, are you married, and I knew he was but that he had forgotten," Theroux says. "I got the uncomfortable feeling I wasn't trying to engage with this person as a human being but trying to expose their frailties. So I had to adapt."

Theroux worked on TV Nation, a satire fronted by documentary maker Michael Moore and he credits Moore for what followed. The BBC screened Louis Theroux's Weird Weekends fbetween 1998 and 2000 and seven When Louis Met... films, including the late Sir Jimmy Savile and an often excruciating encounter with Christine Hamilton and MP husband, Neil.

Does his move into more serious territory suggest he is ashamed of what he is best-known for? "I just follow the subjects I'm interested in," he says. "I'm not trying to acquire a reputation as serious documentary maker for its own sake."

He has remained a dispassionate observer whose ease with all subjects, is evident in the cafe at which he has arrived wearing jeans and a hoodie.

"People say I'm deceptively unassuming but that's the way I go through life," he says. "I'm not flash. You can make it sound calculated but it's pretty much just me. Coincidentally it turns out it can be very effective."

If anyone recognises him in Harlesden, they don't say so. Anyway, his American cousin, Justin, has more than enough glamour for all the family; the actor and director lives in New York with Friends star Jennifer Aniston.