Ask Caroline O'Connor which of the "ladies" she likes best in her show Bombshells, and she claims she adores them all. She loves the way the writer, Joanna Murray-Smith, found a way of penetrating the calm exterior of women's existence and finding what she describes as "their besieged inner lives, the wilderness of the feminine spirit, the wildly volatile territories: erotic, fearful, desperate, cunning". But most of all, O'Connor likes the fact that a show was written especially for her.
"It's about the bombshells that happen in women's lives, such as getting married, becoming a mother or being a teenage prodigy," she says. "Some people have said, 'Here we go again - widows, mothers, abandoned wives...' But the response of the audience tells a different story. One moment they're really laughing, slapping their thighs and exclaiming, 'That's me. How does she know?' and the next, they're in tears."
How much have the ladies been shaped by O'Connor, and how much by Murray-Smith, an Australian? After all, she's an author with a big reputation and has written many plays, including Honour, which had a successful run last year at the National's Cottesloe Theatre, starring Corin Redgrave as the man who leaves his wife of 32 years, played by Dame Eileen Atkins.
"We met just twice, for a couple of hours, on two afternoons, though she'd seen me on stage a lot and knew where my strengths lay," O'Connor says. "I told her stories. I did actually do Shaft at a school talent show, so that part of Mary O'Donnell's story is true."
When O'Connor performs it on stage as O'Donnell, the teenage wannabe who has suddenly to switch her act to Shaft from a Cats number, it is one of the funniest moments in the show. Retaining the sharp feline ears, she adds the chilling glassy-eyed stare of the iconic detective, parodies that outrageous accent, heavy with the implied threat of violence, cocks her fingers in preparation for a gunfight, and would surely crow if she had enough breath.
Another of Bombshells' memorable characters is the dumped wife from Oldham, Tiggy Entwhistle, an abandoned mother and cactophile. "She's gorgeous, though she's not the one everyone talks about," O'Connor says. "Tiggy has joined a cactus appreciation society where she can be alone yet have friends and feel needed. She makes her speech about cacti and succulents being the perfect low-maintenance plants that thrive on their own."
O'Connor was born in Oldham to Irish parents, though the family left for Australia when she was four. She came back to England when she was 17, attending the Royal Ballet School with the intention of pursuing classical ballet as a career. "I loved that kind of martyrdom, the pain, the discipline, but realised pretty well as soon as I arrived there that I probably wouldn't make it," she says. "But that initial physical training has probably kept me going. I went back to Australia, joined the ballet for a year, and then got into musical theatre simply by chance.
"Someone was ill and I did Oklahoma! 'swinging', where you have to learn every role and cover all the ensemble parts. Until then, I'd kept my passion for singing a secret. I was a latchkey child; my parents would be doing shift work and I'd come home, put on an album and sing at the top of my lungs. When I saw Little Voice, I thought, 'Oh my God, that's me!' There was always lots of singing and dancing in our house, but it was the Irish dancing that grabbed me. I took it very seriously and came third in the world Irish dancing championships at the Mansion House in Dublin when a teenager."
O'Connor started working in musicals, and moved to England in 1984 to do Me and My Girl with Emma Thompson and Robert Lindsay. She had no formal training, but claims that the body's vocal muscles are just like any other. "If you use them all the time they become stronger," she says. "I love to 'do' voices - I'm interested in the different sounds, the pitches and the accents - so all those characters from musicals suited me down to the ground." Cabaret, Budgie, Matador and A Chorus Line followed, but it was her performance as Mabel Normand in the London production of Mack and Mabel that earned her a nomination for an Olivier award in 1996.
"I had done Velma Kelly in Chicago and Pam Gems's A Tribute to Piaf. Simon Phillips, the director of Melbourne Theatre Company, and I toyed with other female shows, including monologues by Dario Fo written for Franco Rame. But they didn't feel quite right, so Simon commissioned Joanna. I've always wanted to show that I was versatile, and didn't want to be pigeonholed into musical theatre. I wanted, like my teenage Mary O'Donnell, to challenge myself, even though I'm now 41."
But O'Connor wasn't prepared for the scale of the challenge. "When 70 pages of script arrived from Joanna, they contained no punctuation at all - no full stops or exclamation marks or inverted commas or even paragraphs. Nothing. It was up to me to decide how to voice these women, and when I read the monologue portraying the yuppie, suburban, stressed-out mother Meryl Davenport, I thought, 'This is a nightmare.' I didn't think it was physically possible to perform this woman on stage.
"Joanna, who has three children herself, said to think of doing her as if I wasn't taking a single breath. I knew what she wanted, but the audience wants and needs to respond. You simply have to give them time. When I first did it, they couldn't bear watching this giddy, breathless woman any longer. In fact, when I finished one early performance, a voice from the stalls exclaimed, 'I'm exhausted!' Another woman in the audience, who'd recently had a baby, started lactating, agitated by watching me trying to cope and constantly forgetting where I'd put the baby."
Alternating between these very different women, aged between their mid-teens and mid-sixties, cannot be easy. Apart from the quick costume changes, there are the variations of mood and pace. O'Connor finds that the show's music, composed by the Tashkent-born Australian Elena Kats-Chernin, helps to change the temperature and tempo. "At the end of each monologue you have to switch off that persona. She's done, finished, and like the treadmill on which the mother character constantly runs to try to keep up, there's no time to think what went wrong or what lines the audience didn't get; you've just got to go right into the next character.
"They're a wonderful mix of women, related in subtle ways to each other, and it's from them and in them - the sparks they produce - that I've found the energy. Zoe Struthers is the diva with the baggage and, oh boy, does she have lots of it. Alcoholic, addicted to drugs and making yet another comeback in big bat-wing sleeves and sequins. People are least used to the Australian bride-to-be character, Theresa McTerry, but they become accustomed to her and seem to empathise with what she's got herself into. It's the wit and the irony that distinguish Winsome Webster, the bridge-playing widow. If you don't find humour in this situation, you simply won't last; you'll be incapable of living through it."
It must be lonely, when you're as sociable as O'Connor clearly is, doing a one-woman show. "Well, it's not a play, as I have no one to play with," she says. "I suppose it's as close to stand-up as I'm ever going to get. I can't sit and discuss lines or scenes, and, yes, it's sometimes terrifying and overwhelming. But I'm really close to my ladies. I listen to them, I talk in their voices while I'm getting made up, and they become each other's audience."
She and her saxophone-playing husband have homes in London and Australia, which is fortunate as she's just been booked by the English National Opera to play Brunhilde, the quirky cab-driver in Bernstein's On the Town - rehearsals start at the end of January. After her success as Nini Legs-in-the-Air in Baz Luhrmann's Moulin Rouge!, her next film role was as Ethel Merman singing "Anything Goes" in the Cole Porter movie De-Lovely with Kevin Kline, to be released here this autumn. She will then reprise Bombshells in Queensland before applying her talent for bringing a character to a larger-than-life existence, and her belting voice, to Peter Quilter's show on the life of Judy Garland, which opens in Australia next August. But she'll still be shadowed by her ladies, and she'd like to take them to America next.
'Bombshells', Arts Theatre, London WC2 (020-7836 3334), to 30 October