The improbable story - part fantasy, part fairy tale - goes something like this. As a teenager, I saw a film that entranced me. The plot was simple: it's Prohibition time and two unemployed jazz musicians (Tony Curtis and Jack Lemmon) are chance witnesses of the Chicago Saint Valentine's Day massacre. Pursued by the mob, they make a rapid escape by disguising themselves as women and joining Sweet Sue and her Syncopaters all-girl band. Almost 20 years later, I abandoned my home and young daughter, hitched up my caravan and ran away to join an Italian circus.

It may be crude to claim I became a circus showgirl because of Some Like It Hot, even though by the time I reached Italy I had seen it more than 20 times. At a gala premiere last week of the new print made for cinema release, film critic Barry Norman declared Marilyn Monroe the undisputed star of the film; without this frothy vision of loveliness, it would be just another 1950s romantic comedy.

But even watching it today, I barely notice Monroe. It's Curtis and Lemmon - Josephine and Daphne - who hold me. They taught me what it meant to be a woman. They still do.

I was 18 when I first saw the film. Then an androgynous student, my uniform was dungarees and a short dark crop, rather like Lemmon's. Some Like It Hot seemed to put forward a thesis that reinforced the lessons of my Edinburgh University Women's Group. Women don't exist, they are created. Anyone can become one.

It was many years before I put that lesson into practice. I tried Biba blue eyeshadow, a blonde bob and skimpy Lycra dresses at mid-80s discos. Later I attempted, but never got a grip on, foundation cream and underwired bras. But the circus offered a chance truly to transform. In the ring, the extremities of womanhood could be acted out as in no other place.

I chose an Italian circus because I knew that, so far from home, I could not be unmasked. If I had joined Billy Smart's, touring Britain, someone who knew me might turn up in the audience and my deception would be discovered. "She's not a showgirl," they'd shout. "She's just Dea Birkett in a sequined G-string. She's a fake," not realising fakery was my purpose.

In the 1959 film, we never see how Curtis and Lemmon become women. One minute they're in jacket and trousers, the next in lipstick and stockings. The only time male and female mix is when Lemmon is dressed as Daphne, all made-up but without his wig. It's a disconcerting moment. He's neither male nor female; it's as if he's deciding which he will become. There's a knock on the door. He pulls on his wig. We are relieved. We're in safe - all-female - territory again, sure of where and what we are.

Yet it is the transformation, rather than the finished product, that is the most crucial element. It is also the most secret. Women rarely put on make-up in front of men. It's important to perpetuate the deceit that our femininity isn't a construction at all - we were born that way.

In the circus showgirl's dressing room - an old railway carriage - no men were allowed. One wall was lined with lockers containing everything we needed to become a woman: scant sequined costumes, impossibly long wigs, piles of putty-coloured tights. The other was nothing but mirrors. We could watch ourselves being transformed.

First we had to strip, shed any remnant of who we were. "Ignore your face," said the Bulgarian elephant girl, watching me trace liner around my thin lips. "This has nothing to do with what you're like. You can be anything. Just paint on the face you want."

She guided me through the steps to becoming a showgirl. My pale skin became a yellowed tan. My poppy eyes were made almond-shaped by cats' whiskers drawn from the corners. My thin eyebrows were thickened; false eyelashes were applied. My lips were full and pouting, my short hair hidden under a tight skullcap, topped with ostrich feathers exploding from a diamanté crown.

And yet, despite the immense effort, it was a fragile disguise. A tender finger traced across my cheek and my artifice would fall apart.

Next, attention had to be paid to my body. My legs were squeezed into three pairs of tights, each more supportive than the last, until they curved where they had bulged and were smooth where they had been dimpled. These were topped with fishnets. Becoming a woman wasn't easy. It took a full two hours to get ready for just five minutes in the ring.

Everything about me was indelibly dated: the tan of my foundation, the spidery false eyelashes, the dark beige fishnets. I could have been in a 1950s film. I could have been in Some Like It Hot. But even contemporary female impersonators - Edna Everidge, Lily Savage, Mrs Merton - portray women rarely, if ever, still seen on the street.

It wasn't only how I looked that had to be altered - how I moved had to be learned. Lemmon, as Daphne failing to conquer high heels, describes women's movements as: "Jello on springs ... Is there some sort of motor or something?" adding wryly: "I tell you, it's a whole different sex."

I had to learn to keep my mouth open as I smiled and waved, to show the white of my teeth. My chest and bottom had to be thrust out, making my arched body an S. Gallina, the Russian Hula Hoop artiste, used to practise smiling and waving in the dressing room mirror before she went on, greeting her own reflection as if seeing it for the first time.

Some friends were appalled that I had abandoned my androgynous attire to become a showgirl, conniving in such sexist posturing. But we all, in little ways, contrive at being female. We try out being femme, put on a dress when we rarely wear one, perm and dye our hair, and stand in front of a full-length mirror, wondering at ourselves. We've been doing it since we were small girls. Because that is what being a woman is all about, making yourself up as one.

I was playing at being a woman. Becoming a showgirl just threw that make-believe into sharp relief because it was so blatant. I was exposing the construction of femininity, not conforming to it. I was as subversive as Jack Lemmon playing Daphne.

And still now, watching Some Like It Hot in my combats, with a can of Carlsberg, legs akimbo on the sofa, I long to return to the circus. Impersonating a woman was so much fun.

• Some Like It Hot goes on release on Friday.