Ever since she won the public vote as the frumpy misfit in Muriel's Wedding, Toni Collette has played a series of underdogs and nonconformists. Now her latest role, created by Armistead Maupin, is taking her into darker territory. But just how close to her own life is her screen persona? Sanjiv Bhattacharya meets Hollywood's most intriguing leading lady

Toni Collette once told an interviewer: 'I used to do things to get attention when I was little.' She was pretty effective, too - aged 11, she faked appendicitis so convincingly, the doctors actually removed her appendix. 'My mother had hers taken out at the same age, so that's how it entered my brain. And she told me that when the doctor presses in, that's not when it hurts, it's when the hand's taken away. So I knew when to react.'

It's an extraordinary anecdote by any standard, particularly for an actress. But for Collette, the story is doubly poignant because in her latest film, The Night Listener - an intense psychological thriller starring Robin Williams - she plays a character called Donna who would relate better than most to her 'appendicitis'. Donna, like the younger Collette, also fabricates ailments to get attention. She just takes it a lot further.

The coincidence is uncanny. It's like discovering that prior to becoming an actor Robert De Niro used to drive a New York taxi and sport a mohican. Or that Jack Nicholson was admitted to a mental hospital where he took his fellow patients on a fishing trip. Furthermore, the coincidence is genuine - Patrick Stettler, director of The Night Listener, assures me: 'I know it's amazing how it fits the part, but I swear I cast Toni long before I knew.'

Needless to say, when I meet Collette, in the boardroom of a Manhattan hotel - an oddly corporate setting, but the only muzak-free place we could find - I have no end of questions on the subject. How was her secret revealed? What did her parents make of it? Why did she do it?

'Oh, I don't know why,' she shrugs, cheerfully, with a sing-song Sydney accent. 'It was a long time ago.' She looks tanned and happy in a summery blue dress and a pair of what she calls her 'lesbian Birkenstocks'. Her smile is warm and toothy.

Weren't you afraid of getting cut open?

'I honestly don't know. I'm not that person any more and I don't have any more insight into it now than I did then. I was just a child having a go at something - having a crack.'

Did it turn out the way you expected?

'Look,' she says, her smile fading. 'It was just something I did. I don't want to focus on this, please. Let's move on.'

It's hard to hide my disappointment, but Collette's reticence is understandable. Clearly she doesn't want to invite too close a comparison between herself and a character who is so crackers she's really quite frightening. Donna was inspired by a real woman who contacted the New York writer, Armistead Maupin, of Tales of the City fame. Maupin wrote the novel of The Night Listener, replacing himself with Gabriel Noone (Robin Williams), a late-night radio storyteller who receives a harrowing book manuscript written by a 14-year-old boy. The book details a terrifying history of abuse and torture by the boy's former family, but when Noone contacts him through his adoptive mother, Donna, he begins to doubt the story and suspects the mother of foul play. As the intrigue develops, Collette portrays Donna as desperately vulnerable, yet also reminiscent of Kathy Bates in Misery - creepy, unhinged and dangerously fixated on a storyteller.

Not that Collette quite sees it that way.

'Creepy?' says Collette, surprised. 'No, I see her as a very needy, very sad and lonely person.

'My biggest fear was that Donna would be turned into a monster,' says the director, Stettler. 'But Toni gives her a sense of organic reality. And she does that in everything I've seen her in. That's why she was top of my list - I cast her even before Robin Williams. She just has this gift for inhabiting characters without ever worrying about the effects they're supposed to have or how she's going to be perceived. She's always fully committed. I just think she's one of the best actors in America.'

But she lives in Sydney.

'I meant, one of the best actors in the world.'

Such high praise might sound overdone, but Collette is no stranger to compliments. Ever since her break-out role as the Abba-obsessed misfit in Muriel's Wedding, every director she's ever worked with has rhapsodised over her talent - from M Night Shyamalan on The Sixth Sense to Curtis Hanson on In Her Shoes. She received an Oscar nomination for The Sixth Sense, and now, at 33, is probably at the top of her game - at least the equal of starrier Aussie peers such as Naomi Watts, Nicole Kidman and Cate Blanchett.

'She's very instinctive,' says Stettler. 'There's no great method to what she does - you don't have to talk to her in character when she's off camera or anything. She just simplifies everything.'

The Night Listener required her to play a blind woman for the first time. But instead of glueing her eyes shut as Jamie Foxx did for Ray, or wandering about the set blindfold between takes, she says, 'I just used my imagination, I guess. I arrived in Manhattan, we had four days of rehearsal, and I was completely jet-lagged and I had a cold. So even though I'd made an appointment at a blind centre downtown, I was so out of it, I didn't get there.' Shrug. 'Anyway, my character's a bit of an actor herself, so if she was feeling her way, why shouldn't I?'

Though Donna was the first blind person she's played, she's one of several memorable mums Collette has played over the years. In The Sixth Sense, she was the fraught single mother of Haley Joel Osment who memorably saw dead people. In About a Boy she played the depressed single mother of a 12-year-old school misfit. And this year, in the hilarious Little Miss Sunshine, she is the exhausted mum in a family just bursting with oddballs and neurotics. Notably, both About a Boy and Little Miss Sunshine end with children embarrassing themselves in talent shows only to be joined on stage, mid-agony, by family members who embarrass themselves in solidarity. As a finale, it works the tear glands with a wrench.

It seems there's a theme here - mums and misfits. Does she warm to these kinds of stories?

'Oh God, no, when I read a script I either love it or not,' she says. 'It's an instinctual response from my gut and it bypasses any analytical game I might play. I just look for things that are honest and true.'

Like mums and misfits? 'Well not just mums, but yes, there are a couple of themes that keep rearing their ugly heads!' She laughs. 'One is acceptance of change, because life is change. And the other, I guess, is acceptance of self - people who learn to accept even the ugliest or scariest parts of themselves. That's what Little Miss Sunshine is about. Because ultimately life is about connecting with yourself and then being able to connect with everyone around you. The better you know yourself, the better your relationship with the rest of the world.'

Collette's given to these kinds of pseudo-spiritual generalisations. She believes in the power of the planets, and she has a Buddhist tendency to prize instinct over analysis. If you give her an opening, she'll happily chat away about how 'we're all part of each other' and 'we can't dominate nature because we're part of it, you know?' But she's a little more guarded about how this all relates to her own life. Her journey to this point has had its share of turbulence, punctuated along the way by panic attacks, bulimia and a curative course of therapy and meditation. 'I think they were an example of the negative effects that fame has had on my life,' she says carefully. 'I don't look back on those experiences with fondness.'

The upheaval began with Muriel's Wedding in 1994, for which Collette had to put on 40lb in seven weeks. Not only did the experience leave her battling with bulimia afterwards, but the film was such a success, it catapulted both her and her co-star Rachel Griffiths to worldwide stardom. At 21, Collette was thrust from suburban normality in Sydney to a life of champagne, celebrity and first-class air travel.

'I remember going to a screening of Muriel's Wedding at a film festival on the Gold Coast of Australia,' she says, 'and when they took me up to my room... it was probably a really tacky hotel, looking back, but I had the penthouse. So I was doing tumbles on the carpet and opening the champagne and looking out at the sea saying, you know, "This ain't Kansas any more!"'

Suddenly this was the norm. 'It was all posh dinners and money and having to talk to all these fabulous people and form opinions about things I really hadn't paid any attention to before,' she says. 'It's a cliche, but I had to grow up very quickly.' There were men, of course - notably Jonathan Rhys Meyers, five years her junior, whom she met on the set of Velvet Goldmine. The work kept coming, as did the accolades, not least the Oscar nomination in 2000 for The Sixth Sense. 'Of course I wasn't disappointed when I didn't win,' she says, grinning. 'I didn't want to get up and speak in front of all those people. I was shitting myself!'

But she kept moving. Within six years she bought properties in Manhattan, Brixton and Los Angeles, but every time she sold up and moved on for one reason or another - the weather, the smog, the people. 'So I was literally living out of a suitcase most of the time,' she says. 'I had so much shit in storage, all over the planet. It was a crazy way to live. Rachel [Griffiths] went through a similar thing and we used to collide in different cities around the world. I'm so glad it's all over.'

But it sounds like a blast!

'It was a blast. But you can't blast on forever!'

She was 28 when she finally settled down. 'It was a natural sigh. You know people talk about the Saturn return every seven years? Anyway, it's meant to happen when you turn 28, and it did. It was like "Bing!" I was over it. I realised that I felt most at home in Sydney where I grew up, and that was it. I moved back there and bought a house. There was no big heave-ho. It all happened very quickly.'

Other factors contributed to her move to Sydney. When she split with Rhys Meyers after a relationship of only a year, she'd suffered several months of panic attacks. And the jet-set life had been taking its toll - she wasn't particularly enjoying her success any more, a problem that therapy went some way to fixing. But like her appendicitis, she doesn't want to talk about that today. 'Sometimes life hits you on the head with a saucepan,' she says cryptically. 'But I'm not here to talk about saucepans.'

Certainly she has managed to slow down since then, settling down with a musician called Dave Galafassi whom she married in 2003. 'I knew immediately he was the one,' she says, beaming. Their wedding featured Tibetan monks chanting as they walked down the aisle. And domestic bliss has given her newfound appreciation for the life she led during the whirl of her twenties. 'Travelling's so much better when you know you've got a lovely home to go back to,' she says. The same goes for her career. 'I can just enjoy it for what it is, without depending on it too much. I used to rely on my career as a way to express myself. I was a shitty communicator.'

One thing that hasn't changed is her distinction between being an actor and a celebrity. 'I've always been a working actor,' she says. 'Big difference. I'm not interested in promoting myself or being famous. Don't get me wrong, I like getting tables at restaurants that have been booked out for months. But I don't want people to identify with me instead of the character I'm playing.'

Collette's schedule is crammed for months on end, a prospect that fills her with delight. 'On a good day, everything just falls away between action and cut, and that feels amazing,' she says. 'When you're not thinking, just doing.' But beyond the acting, Collette sees herself directing, as do so many of her peers. She also has her heart set on making a silent movie. And then there's the music. Ever since she started out making musicals in high school, Collette has sung and written songs. This year, her first album comes out, entitled Beautiful Awkward Pictures

'I've always wanted to make music, and now, after everything I've been through, I'm finally in a place to do it,' she says. 'I think you get what you need in life. Your subconscious thoughts affect your day so deeply that if you actually chose to focus on something, it would probably happen eventually.

As the lift doors open, she holds out her hand to shake, with that big toothy smile again. 'Have a good one!'

· Little Miss Sunshine opens 8 September and The Night Listener on 15 September