Investigating Gillian

The only residue of those dark days seems to be a mental and physical restlessness. She finds it hard to sit still, forever leaping up and down to answer phones and mop up the coffee she's spilt.

The X-Files' punishing filming regime has left her formidably driven. She is rehearsing five days a week for What the Night is For, a new play by Michael Weller, but finds the schedule alarmingly slack. So to occupy herself, and quell her nerves about her West End debut, she uses her spare time to adapt a novel into a screenplay that she will eventually direct. She's currently coproducing something else that she won't name, and there's also a small British film in the pipeline.

In addition, she's somehow found the time to gut the London house which she bought this year. The Moroccan theme is her own idea. The builders have been here for ten months, though she thought they were staying for three. It's most householders' nightmare, but this sort of creative chaos makes Anderson feel right at home. 'I have to keep my mind going the way it has been over the past ten years. It has become a habit now, I can't stop.'

The expensive redecoration doesn't even mean she's going to keep her house once the play's run ends. She has other houses in Vancouver and Malibu, and enjoys shuttling between them. 'I'm not so attached to this place that I couldn't say tomorrow I'd rather be somewhere else. I like to live out of a suitcase.'

Anderson doubtless owes this rootlessness to her peripatetic childhood. Born in Chicago, she was just a few months old when her American parents shipped the family off to Puerto Rico to stay with her father's parents so he could save up money to go back to college. Her earliest memory, she says, is of gathering eggs in her nappy under the blazing sun.

When she was two, her parents moved her to London so that her father could attend film school (he went on to make commercials for a living). She was brought up in Crouch End and says she feels 'very much at home' here. Her accent is so English you notice immediately when an Americanism slips in; she even likes Marmite ('in small doses'), an almost infallible test of Anglophilia.

It came as a tremendous shock when her parents moved back to America, to Grand Rapids, Michigan, just as she was approaching adolescence. 'Small-town Middle America compared to London --it's like, slash your wrists,' she says. Her brother and sister were born there. Anderson speaks fondly of her siblings, but after an entire childhood alone, suddenly having to share her parents must have been very difficult. In classic teenage fashion, she sought attention through rebellion.

Voted 'Most Likely to be Arrested' by her classmates, she proved their predictions correct by being flung in jail on graduation night, after she was caught trying to glue the school's locks shut.

'My boyfriend at the time, who was about ten years older, just went home and had a beer,' she remembers. 'I was let out early in the morning in my prom dress. It was hysterical.'

Her parents did their best to cope with their suddenly errant daughter. 'Bless them,' she says fondly, if a touch patronisingly. 'They were very kind. They got nervous a few times when I showed up at the dinner table after just shaving off half my hair, or when I got my nose pierced

after my dad told me not to, and I had to walk around the house sideways so he wouldn't see it. Then there were the boyfriends who were drug addicts, stealing food out of the cupboards... I think I put them through a bit of a rough time.'

If so, it was mutual. Anderson clearly has mixed feelings about her upbringing, and has said her parents 'didn't do the greatest

'There were difficult situations to be in as a child,' she says, though she doesn't want to go further. 'You're born with that thing of keeping going against all odds.'

Anderson went to college in Chicago then moved to New York, where she waited tables in between stints on stage. She followed a boyfriend to Los Angeles, sold her return ticket and spent a year going to films, working-out and partying.

And then she went up for The X-Files job. 'I wasn't in the habit of auditioning for TV,' she says. 'I had very elitist views. I've never been a TV watcher and I don't particularly like it. But after being out of work for a year, I thought, "Let me just go up for somethingî.' The auditions went on for days: 'A dreadful process.' She finally found out she had the role on a Thursday; by Saturday, she was flying to Canada, being paid 'money that I'd never heard of before'. (By the end of The X-Files, she was earning £3 million a series.)

Anderson's life was never the same again. When she returned to America, she found herself a star. 'It was very bizarre. We'd go to the award shows and there would be 200 photographers yelling my name. There are all these pictures of me like a deer caught in the headlights.' But most of the time, she was in the 'rain and the snow and the sleet and the woods' of Vancouver, filming for 80 hours a week, nine months of the year.

'We'd work all week, till sunrise on Saturday morning. Sometimes it was fun. At 3am, you get slap-happy. And sometimes you feel like you're going to die and you can't possibly do another hour. You just have to take it one day at a time.' A visitor to the set remembers it as 'very macho. I thought David Duchovny treated her like she was irrelevant and hysterical.'

It's not surprising that there was friction between her and her co-star Duchovny. 'For all intents and purposes, we had a marriage,' she says now. 'It was very intense.'

Meanwhile, she had fallen in love with Klotz. They married on 1 January 1994 and their daughter Piper was born the same year. Two years later the marriage was over, blamed by Klotz on Anderson's ill-fated friendship with X-Files extra Adrian Hughes, who turned out to be a serial sex attacker. A year-long relationship with another X-Files actor, Rodney Rowland, also ended.

'But bad experiences have definitely taught me things about myself and my patterns with men,' she says. 'It's all part of the lesson. The trick is not to make the same mistakes over and over.'

She has been single since March, the longest time she's been alone. Is she seeing someone now? She smiles secretively. 'Perhaps... no, not perhaps. Let's say no.' I don't believe her.

Klotz still lives in Vancouver, and Piper, eight, shuttles between her parents on a monthly basis. 'If children are loved in two households equally, a split home doesn't have to be devastating,' says Anderson. 'And when the husband and wife split, when they get out of the suffocating situation, they both find they are happier.' She has developed 'survival mechanisms' for coping with her daughter's absences. 'It's very difficult for the first few days and then I settle in. But every once in a while I get these huge pangs.'

What the Night is For treats the same themes of conflict between love and duty. Anderson plays a middle-aged married woman who meets up with her ex-lover. Should she stay with her unsatisfying spouse, or leave him for a chance of happiness?

It's an emotionally intense play that will test Anderson's skills to the limit. There's no doubt that she can act - her performance as Lily Bart in the film of Edith Wharton's The House of Mirth received much critical acclaim - but can she sustain an emotionally draining performance night after night, on stage opposite just one other actor?

'Everyone who sits in front of me expresses their own fears of being in that situation,' she says, thoughtfully. 'It's making me stronger, pushing away that fear. I'll be better at dealing with it when I really need to be afraid.' And for one minute, she sounds just like Dana Scully.

What the Night is For is at the Comedy Theatre, Panton Street, SW1 (020 7369 1731), from 7 November