"So," says Jane Treays casually, during a break in our interview, "how do you see the next 10 years of your life panning out?" This is not the kind of question a journalist often gets asked. Most interviewees inquire about quotes, or copy approval, or whether you can claim the drinks on expenses. But it is Jane Treays' ability to pose such direct, discombobulating queries, questions to which we all secretly long to respond, that make her one of Britain's most successful documentary-makers.
In her latest trio of films, starting next Monday on Channel 4, this sympathetic, non-judgemental probing is deployed with exceptional effect in her studies of people on the edge. Previously she has peered into the lives of flashers, male prostitutes, models, rock stars and polygamists. She made an astonishing portrait of the conductor Clive Wearing, left by illness with a memory of seven seconds.
Her latest series, A Child's Life, continues her fascination with the marginalised and misunderstood. The first film is about primordial dwarfs, the second about child carers, the third about children whose parents have committed suicide.
"I've always been passionate about communities that live on the edge and are unspoken for," she says. "With the child carers, I was wondering how you could be normal if you were having all these responsibilities - with one family, it was like going back into another world. Their situation was almost Dickensian and really tugged at the heartstrings."
Lavishly shot, lit like oil paintings, her films feature lingering, uncut close-ups of the subjects' faces as she questions them off camera. Some of the scenes of children talking about their dead fathers, or reading their suicide notes, are almost unbearable for their unflinching intimacy. "My dad killed himself and his dad killed himself," says one boy. "I hope it doesn't run in families."
Treays is everything a charming, polite, middle-class Englishwoman should be. But there is plainly something about her, something deeply un-English, that makes people want to unburden themselves.
Of the suicide film, she says: "I don't think these children had ever been talked to like that before. I sit close to them and never take my eyes off them, and I keep the questions very simple. I never prepare or write anything down - I just watch and listen. My passion is to listen to stories and watch how the light falls on people's faces."
What with prostitutes, polygamists, flashers and dwarfs, there is no denying that her portfolio focuses on the freakish. But Treays insists that she is looking for the ordinary in the extraordinary: "It struck me at an early age that you could learn a great deal by people who either choose to live differently or have to because of the way they look, and I look for the connections as people that we have with them."
Treays' work underlines the strength of the documentary as a television format. It's a puzzle why viewers don't know her name as well as they know those of Molly Dineen or Nick Broomfield. Yet she is concerned by certain trends affecting the format, chiefly decreasing budgets and the emphasis on the confrontational. "It's very alarming. Young film-makers are being sent out with little cameras to difficult situations without the support of a crew. They do what is called 'hosing down' the subject, which means they film so much stuff that when the subject is tired they'll confess to something. While I think Wife Swap is very interesting on one level, and occasionally brilliant, they do deliberately put in opposites to create tension, so it's not what's really going on in people's lives."
But can observational documentaries reflect any better the truth of everyday lives? "It's a huge question. What is reality? Are they different when I'm not there? I cope with it by literally making it my view of the situation. I write it, direct it, research it, and I ask very deep questions: 'What does that mean to you? What happened next? What are your dreams?' The films are like moments in time, because they are what happens on a particular day when I'm there."
Treays has regretfully stopped shooting on film because of the cost, but the tapes - shot by Steve Robinson - are put through a film effect to soften them. The result is very different from the mass of documentaries shot with digital video cameras.
As a divorced mother of a teenage girl and boy, she believes that personal experience has helped her as a film-maker. "I got better at it when I had children and I've got better since I've been divorced because of the sadness I felt. If I'd never experienced pain or sadness I would find it hard to identify. Documentary-making is an area people get better at as they get older. You have to know yourself. You have to be very tender in your relationship with the person you're making a film about."
One of Treays' most memorable films was Painted Babies (1996) about child beauty pageants in America. After developments in the JonBenet Ramsey case, the BBC is remaking it. Her next two-year project returns to the domain of the socially reviled - in this case teenagers who sexually abuse other children.
Even as she tells me this, she laughs. "I know! I'm a bit of a tortured soul and I go and meet tortured souls so we're probably not everybody's cup of tea. But I think at the end of it, everything I do is made with love and is about love in its many different forms and perversions."
'A Child's Life' begins on Channel 4 next Monday at 9pm