People think they know Billie Piper. They see her great gaping smile and her wide-open guileless face and imagine that she's just as sunny and uncomplicated in private as she appears to be in public.

Except that today she's not like that at all. In fact, she's so nervous she can barely talk, pacing about with her arms crossed, manically chewing gum and puffing on Marlboro Lights.

With only a few hours to go before she makes her stage debut in Christopher Hampton's play Treats, (about a love triangle) in Windsor, she is, she admits, feeling more tense than she has ever felt before.

So much so that we decide to postpone the interview. A few days - and some good reviews - later, she's feeling much calmer.

But in her heavy-framed glasses and her baggy green cardigan, she still cuts a surprisingly intense figure as she sits on one end of a beaten-up sofa in her bunker-like dressing-room.

She's friendly and obliging, but also wary: a naturally trusting person who's been bashed so many times that she has learnt to hide her real self away from outsiders.

Perhaps this is inevitable. Still only 24, Piper has crammed a remarkable amount into her life so far.

It's nine years since she had her first number-one single, six years since she married Chris Evans and only just over three years since she became an actress.

Occasionally, she says, she has a job believing it all herself. In one sense she's got everything she ever wanted: success, respect, fame.

But in another sense success was nothing like how she imagined it to be. Far from bringing her happiness, it made her anorexic, suicidal and frightened that she was losing her mind.

'For a long time I felt like I was just a pawn in other's people games; everything was plotted out for me and basically I just did what I was told. It's only recently that I've stopped feeling like a complete charlatan and sorted myself out and found my self-esteem again.'

But, to begin with at least, lack of self-esteem was never a problem for Billie Piper. She had huge, apparently inexhaustible supplies of the stuff, along with abnormal levels of self-possession.

A 'really ballsy child' in her own description, she grew up in Swindon - where her father runs a construction business - and was hell-bent on becoming a success almost from the moment she could walk.

'I know it sounds odd but I very quickly became bored with being a child. I felt suffocated by Swindon and frustrated by being there. What I really wanted to do more than anything was go to London and become an actress.'

At 14 she left Swindon, seldom to return, and landed a part in an advert for the magazine Smash Hits.

All too aptly in view of what happened next, she had to run up to the camera, blow a bubble-gum bubble and shriek, 'Pop!' Within a year Piper had become the youngest singer to debut at number one in the charts, with Because We Want To.

But her euphoria didn't last long. One night in the Top of the Pops studio, she watched as the audience drifted away and the house lights came on and realised it was just a room much like any other.

All the glitz, all the glamour, seemed to shrivel up and vanish. 'Suddenly it all felt like an anticlimax. I watched the Spice Girls stub their cigarettes out into polystyrene cups and they were just like five ordinary girls.

'Everything seemed much smaller somehow, and it struck me that this whole world I'd envisaged wasn't really like that at all.

'I'd thought it would be a lot warmer than it turned out to be. Instead, I saw how much of a business it was and just how much of a cog I was in the wheels of that business - and that really freaked me out. The ridiculous thing was that I'd never wanted to be a pop singer in the first place.

'I liked being on stage, although I never liked the publicity - having to go round the world talking about myself. I used to feel such a phoney.'

She could have turned to her parents, of course - except that she was no longer on speaking terms with them. The split was prompted by their selling a story about her to a tabloid, but in retrospect, Piper says, their relationship had been falling apart for some time.

' I made them redundant as parents because I thought I didn't need them. When I was 15 I'd go, "Mum, I've got a mortgage and a boyfriend. I can handle it." Of course, I did need them, but I pushed them further and further away.'

As she talks about her parents, all at once Piper's voice cracks and her jaw - that unforgettable jaw - starts to quiver.

'That's the thing I feel most guilty about, shutting them out,' she says. 'I would call them when I was in despair and say, "Can you come and get me?" And they'd always be there. But then they wouldn't hear from me for weeks. Now I can see that they were as much victims of everything as I was.'

By the age of 18 Billie Piper was all washed-up as a pop star. She was also pretty washed-up as a human being.

'I used to cry myself to sleep every night - crying about the mess I'd made of my life. I also realised that I'd become what I most despised. I was vain and controlling and also I had this terrible eating disorder. It all made me feel like shit. I couldn't take care of myself at all and that frustrated me so much.

'I was a jumped-up young girl, full of neuroses and a bit mad. And all the time I had these voices of self-doubt niggling away in my head telling me that I couldn't hack it, that I didn't have the voice of Mariah Carey and that I had no business pretending to be a musician.'

Her salvation came in the implausible shape of Chris Evans. He wooed her with a silver Ferrari covered in roses, whisked her off to Las Vegas to be married and helped her regain control of her life.

'Chris taught me how to take care of people and how to take care of myself. I've always felt very self-conscious about missing out on my education and he advised me what books to read and movies to see. Of course, there was a lot of pissing about, too, but he held my hand and that was all I wanted.'

Their life together, she says now, was like 'one dream sequence followed by another'. Yet while an air of unreality may have hung over everything they did, the emotions, she insists, were real enough.

'I loved him and he loved me and that was all very genuine. But we went from being inseparable to spending a lot of time apart for work - it was a real shock to the system. As lovers we realised it wasn't going to work and that we were better off as friends, but we're still very close - soulmates, really.'

It was while she was married to Chris Evans that Piper started taking acting lessons in Los Angeles.

'I didn't have a lot of confidence left by then, but I did have a lot of conviction, if that makes any sense. I was determined to make it work.'

Within a matter of months she landed her first big acting job - alongside James Nesbitt in a contemporary adaptation of Chaucer's The Miller's Tale. Less than a year later she landed the part of Rose in Doctor Who.

By then, though, a number of things had changed, most notably her attitude towards fame.

'I was - I am - no longer that bothered about being famous. All those dreams I'd had about being a big film star, or winning an Oscar - I've let go of them. Obviously, I want the things I do to be enjoyed, but I'm not worried about going to Hollywood or making serious money. '

But, however much her attitude may have changed, she still has the trappings of fame to contend with - the paparazzi, the press intrusion, the insistent fans.

'I don't do the whole showbiz bullshit any more - being seen in the right places with the right people. On the other hand, when I was a singer I became really reclusive and that nearly sent me mad, so I don't want to make that mistake again.

'Now I just try to lead as normal a life as possible. Sometimes it does piss me off when men with cameras start following me down the street. I can feel this bile rising in my stomach and I hate the whole thing. But it's something you just have to learn to deal with.'

Like a lot of people who have never had a proper childhood, there's something curiously childlike about Piper. You get the sense she can do adulthood very competently but without entirely believing in it herself.

Yet for all her uncertainties, life has plainly improved of late. Recently, she's patched up her relationship with her parents. 'I've said sorry to them for all the hurt I caused, which was very important to me.'

Since her split from Evans - she refused to take any money from him in their divorce settlement - she's been out with a childhood friend, Amadu Sowe, and is now reported to be dating her Treats co-star, Laurence Fox.

As for the work, it keeps rolling in - Treats will tour Bath and Richmond on its way to the West End and she'll soon be seen in an ITV adaptation of Jane Austen's Mansfield Park.

Her decision to leave Doctor Who last year was something she agonised over for ages. 'I loved it so much, but I knew that if I didn't go then, I would have always been scared to leave, and then I wouldn't have tried other things.

'Of course,' she says pulling her green cardigan around her, 'it could still turn out to be a really bad choice. But somehow I think it's going to be OK.'

Source: Stella Magazine