Ian Burrell on Louis Theroux

The spectre of the hacking scandal means somejournalists face a real prospect of spending time in jail. They'd be advised not to buy Louis Theroux's new DVD.

Theroux is a reporter for whom the very worst of prison cells are a source not of fear but of fascination. In one of his more recent expeditions, he spent days walking the landings of the Miami County jail, directing his bespectacled gaze through the bars at the murderers and innocent men forced to live cheek by jowl at up to 23 to a cell. The gawky Englishman in a checked shirt seemed to fascinate the killers and give comfort to the fearful among an inmate population of 6,000, some of whom had not even been charged.

He probably wouldn't last a minute among the thugs and gangsters but – as he is big enough to readily admit – Theroux never works alone. It partly explains why such a shambling and awkward figure manages to get access to such high security buildings in the first place (he previously reported from inside the notorious San Quentin State Prison in California).

"People think well how can he be as nervous, shambling – or maybe I mean bumbling – as he looks and still make those programmes," he says. "What they forget is that the shows are collaborations. There's this support team of director, associate producer, series producer, executive producer, editor. They are group endeavours and in a way I am able to be more bumbling because someone else is doing most of the real work."

Even so, beneath that bumbling exterior is a "tenacious journalist", he asserts. It's not an idle boast from someone who has made some of the BBC's more memorable recent current affairs programming, shining a light on Ku Klux Klan civil rights activists, Amazon Avon ladies and Ultra-Zionists in Israel.

The heavy subject matter of documentaries such as Miami Mega Jail, means Theroux sometimes needs to inject a little humour. So after days of being warned by cons that the first rule of the prison was never inform, the presenter casually asked the baddest man in the jail to say on camera who had been responsible for the previous night's stabbing. "I had been told 10 times, 'Don't snitch, never snitch, snitches get stitches, you get stabbed if you're a snitch'," he remembers. "Some people might call that faux naive but it's just a joke really. I think the viewers are savvy enough to know."

As well as considering himself tenacious, Theroux is reluctant to be seen as a geek hero. "For publicity purposes everything gets simplified and the fact that I wear glasses and am somewhat bookish makes me a geek. That's fine, there needs to be a shorthand, but there are important geek traits that I don't really share. I'm not especially tech savvy," he says, and then accidentally knocks his coffee over, just like a geek.

There are younger broadcasters who seem to have inherited his style, such as the amusing Yorkshireman Alex Riley, drawing out truths from their interviewees by adopting an informal approach. It's sobering to think that Theroux, 41, is a veteran now, having been on the BBC for 13 years. Although he acknowledges the work of Michael Moore, who gave him an early break, and Alan Whicker, who blazed the trail of quirky documentary, he doesn't spend much time thinking about other presenters. "I didn't get into this to be successful. I'm following my interests and there's something about investigating the world and creating a watchable entertaining programme out of it that is deeply satisfying."

With journalism in the dock, how authentic is his oddball style of reporting? Theroux accepts that finding a balance between entertainment and revelation is a challenge. "We have a double agenda of trying to deliver something exciting that people will talk about and will brighten their day and will amaze people and make us proud to have created an object of beauty. And on the other hand being true to the story," he says.

"Those interests are not always or necessarily in alignment. You could say that by putting a geeky guy in glasses in one of the most volatile cells in a Miami jail is kind of chumming the waters and done in a certain way could seem like sensationalism. But on the other hand it's part of the truth."

Perhaps the geek badge is something he's not so uncomfortable with after all. All of his programmes are about "uncovering something", he insists, and it's no hindrance to be naturally obsessive about detail and to ask endless questions. "It's a foible of mine that I over-interrogate," says Theroux. "It's a little bit of an issue in my personal life that I can sometimes just endlessly analyse myself and my surroundings in a way that's destructive. But when it's turned on an institution that deserves to be investigated it's a huge professional advantage I think."

Louis Theroux: The Odd, The Bad and the Godly is out on 15 August