It is not often that the theatre sees a genuinely dazzling debut, but Tara Fitzgerald last Wednesday woke up to the sort of reviews she might have written herself. The collective critical nostrils clearly scented a star in the making during her performance in Our Song, the play by KeithWaterhouse which opened at the Apollo Theatre on Tuesday night.

It is not as though she has been struggling away in the provinces before she got this West End break. Extraordinarily, the opening of Our Song was the first time that she had ever walked on stage in front of a paying audience. Aged 24, two years out of drama school and with limited television experience, she is now nightly holding her own against one of the profession's most daunting listed ruins, Peter O'Toole. She is strikingly charismatic, which has helped. Her vibrant personality comes with brunette, Kate Nelligan-ish looks; her voice is a sexy baritone. One critic commented on her as a femme fatale, which Tara defines as a woman who smokes but never has alight. Whatever, matches certainly seem to be the key ingredient in Waterhouse's witty play, in which an advertising executive crosses the generation gap in romantic pursuit of a footloose young thing.

The show is really an adulterous odyssey, Send in the Clowns providing its bitter-sweet theme tune.Since their affair is conducted on a diet of booze, cigarettes and sex, one could justifiably accuse it of being an upmarket replay of Waterhouse's extremely successful Jeffrey Bernard Is Unwell(in which O'Toole also starred), only this time with champagne and luscious crumpet. The last few weeks have been a whirl for Tara Fitzgerald, bringing the show from Bath to London. The fact that the theatre business is new to her is demonstrated by her endearing lack of First Night savoir faire. Her bouquets sit around in their cellophane wrappers; her dressing room - she seems to live out of carrier-bags - is reminiscent of a student bedsit.

Refreshingly, she hasn't developed yet that tedious interest in her own inner self that many actresses like to explore in interviews. You get the impression that Tara Fitzgerald would rather have a drink and a laugh. In fact she seems unsure of how to react to her overnight sensation. What about those reviews? 'It's actually a bit like reading about someone else.' she says. She is certainly keeping a scrapbook. 'You wouldn't believe the money I spend on newspapers and magazines - for sentimental reasons. I want to remember what all this has been like in a year's time.'

She may, of course, be out of work by then. Glowing reviews and showbiz awards can be the kiss of death for promising newcomers, though in her case one doubts that will happen. Our Song started life as a novel. Tara Fitzgerald has so far worked on two other modern novels for television -as Polly the twin-fancier in The Camomile Lawn (by Mary Wesley), and as an inebriate seductress in Angus Wilson's Anglo-Saxon Attitudes. She was noted - without her name being remembered - for sporting herself in sex scenes in both, much to the delight of the tabloids. Indeed, nudity has become her television trademark.

'It's a wonderful publicity pitch for the producers,' she says. 'I suppose they've used me, but I've used them too. I don't need to do it. I don't have a Godiva complex or anything!' she explains. She doesn't appear naked in Our Song though she probably wouldn't mind if she had to. Michael Redington argues she is a rare and once-in-a-generation discoveery. Being the show's producer, he would say that. But the evidence of a natural-born talent is in her performance, during which she effortlessly gets through a lavish wardrobe in over a dozen high-speed costume changes. She combines a moody resentfulness towards her prey with a crackling sexual voltage. Above all, she is quite staggeringly self-assured for someone with no stage experience.

'If I am going to fall in love with her, there has to be a reason,' O'Toole is reported to have said when the hunt was on for the right actress. O'Toole, Redington, Waterhouse and the director Ned Sherrin formed a committee to choose the right girl. They auditioned scores of hopefuls. Tara Fitzgerald was shortlisted and attended her fourth audition with a vicious stomach bug. 'I was really green - I could hardly look people in the eye. I felt they all thought I was the actress doing the heroic "I'm not at all well, darlings' bit. Peter recommended I get some charcoal tablets.'

The tablets worked. The avuncular committee instinctively chose her. 'They've been great to work with. Peter has been really encouraging. It's strange to think of all those people who have generated a real chemistry but actually hated each other - Vivien Leigh complaining about Gable's bad breath, that sort of thing. But Peter's wonderfully relaxed to work with. He's like a floppy doll. I think if I had been with nervous or amateurish people, it would have all been very different.'

Was Peter O'Toole, though, her type in real life: 'I don't have a type,' she retorted with inspired diplomacy. Her Anglo-Irish background has many theatrical connections. The Norman Rodway was once her stepfather and her great aunt is Geraldine Fitzgerald (who was in the Olivier film of Wuthering Heights), so she was brought up used to the company of actors at home. Her mother - her domestic anchor in a boyfriend-less life - was, she claims, always convinced of her showbiz aptitude. 'My name means "star" in Sanskrit,' she says, laughing at the notion. One of the interesting things about her performance in Our Song is the obvious empathy she has with the lost, coquettish character she plays. 'I know plenty of people like Angie in the play. She's very like an actress - in that confident, showy way. Like her, I was a great party girl. Nightclubs have that nether world feel; they are for lost people to go to and have an identity for an evening. My drama school changed all that, but that clubby girl thing was me for a while.' The future is more television drama; she has resisted so far the blandishments of Hollywood after her success in the film Hear My Song. She has an American agent, but she says she uses the film scripts sent to her as jotting pads by the phone.'