How a self-confessed geek from Yorkshire became the cool heroine of 'Desperately Seeking Susan'

Alice Jones meets Emma Williams as she prepares to take on the role in a new West End musical

Emma Williams is sipping on a steaming bowl of nutritious, noxious-smelling brown broth and telling me enthusiastically about her latest hobby, "geocaching", a niche outdoor pursuit in which participants use GPS to seek out hidden caskets. "It's treasure hunting for geeks. And I'm very proud of that," she says, a touch defensively. It's not really the kind of thing you can imagine Madonna, or her most fabulous celluloid creation, Susan, spending her weekend doing.

Indeed, aside from a feline flick of black eyeliner and a shock of blond hair, it's hard to see how this fresh-faced 24-year-old Yorkshire girl in a stripy pink and black T-shirt, cropped tracksuit bottoms and matching stripy socks, will ever transform herself into the effortlessly cool heroine of the cult 1985 film Desperately Seeking Susan. Williams has described her take on the role in the new stage musical, which uses Blondie songs as its soundtrack, as a "cross between Debbie Harry and Madonna with influences from Courtney Love". An unholy trinity if ever there was one – so how is she adapting to the sex, drugs and rock'n'roll lifestyle?

"She's very different from me," admits Williams. "I'm so authority-driven it's untrue. I'm the sort of person who doesn't cross a road when the red man is showing because I'm scared I'll get arrested." Nevertheless, a little bit of Susan's insouciant free spirit is starting to rub off on the self-confessed good girl. Williams has adopted a new mantra, WWSD or "What Would Susan Do?", which is, apparently, making her more easy-going. She's even stopped wearing the "lucky ring" that has been a constant companion of her auditioning and performing life over the last 10 years. "A few months ago, I just thought 'do you know what? Susan wouldn't need a lucky ring'."

Williams has some pedigree for the bad girl role, having last appeared on the London stage as Christine Keeler in A Model Girl, a musical based on the Profumo scandal. Otherwise, she remains best known for her award-winning West End turn as Truly Scrumptious in Chitty Chitty Bang Bang. "Susan falls mid-way between the two," says Williams. "Christine was raunchy and sexy and aware of it. Susan is effortlessly sexy – she doesn't try."

In an effort not to imitate Madonna's performance, Williams hasn't watched the film in a year- and-a-half. They are very different beasts – Susan Seidelman's lo-fi movie grew out of New York's underground scene, as did Madonna, and became a quirky, unexpected success. The stage version is a glossy, all-singing, all-dancing hi-tech extravaganza. Its random melding of a poppy film with perennially popular songs is bound to be a box-office hit, but is it contributing to the so-called crisis in the West End?

"I love plays," says Williams. "The problem is that at the moment, people aren't going to see them regardless of whether we put them on or not. If the West End can be given a new lease of life with musicals, then we should embrace that and hopefully that'll mean there's more scope for plays to come in. I think it's a shame that we have so many musicals in the West End that are not new. But that's because we don't take a risk on new theatre very often in this country."

Is casting musicals via a television talent contest the answer? "If we can bring more people into the West End who wouldn't go, then that's a very good thing. We just need to be very careful – let' s put it that way – because these sort of people need to be nurtured and looked after. I was 18 when I did my first show. I had no technical or formal training. I got the dream – I got what they've been trying to give to other people," says Williams. "I slowly built up my stamina and learnt how to make myself last eight shows a week. I took the holiday you're forced to take and I think I only missed five shows due to illness, three of which were because I was having a foot operation and two because I had a cold and I had food poisoning."

If it sounds like Williams is choosing her words carefully, that's because she is. Hired by Andrew Lloyd Webber as the professional stand-in for the role of Maria in The Sound of Music, Williams was due to share the performance schedule with the television series winner Connie Fisher, with each performing a number of shows a week – common practice on the gruelling treadmill of musical theatre. When Fisher expressed a desire to perform all eight shows, Williams left the show. When I suggest that the debacle may well have been a blessing in disguise – if she'd played Maria, she probably wouldn't be rewriting her good girl script now as Susan – Williams remains tight-lipped, a picture of professionalism.

But then she would be professional – she's been performing for most of her young life. A "gawky" child with glasses, Williams was bullied at school. Her godmother – who had been a dancer with the Bluebell Girls and rode an elephant in Billy Smart's circus – suggested that dance classes might give the shy girl poise and grace. Williams never looked back. Before long she was spending her Sunday afternoons watching old Fred Astaire movies on BBC2 and, at the tender age of 13, she played her first role, Peggy in 42nd Street, with the local amateur dramatic group in Halifax. Around the same time she started evening stage school at Stage 84 in Bradford and secured her first television role, in Heartbeat. She then chalked up a steady job per year on other Sunday night staples such as Where the Heart Is until she landed a part in the Steve Coogan film The Parole Officer in 2000. Spending 16 hours a day on set and studying for her five A-levels in her hotel room at night, she began to consider a career in acting.

From there it was a whirlwind couple of months until she got the part of Truly Scrumptious, becoming the youngest ever female to play a lead role in the West End. Her agent had only sent her along to the audition "for the experience". "The Parole Officer came out on 11 August, I got my A-level results around 25 August, I signed to ICM on 11 September, my first audition was on 31 October and I got Chitty on 10 November," she recalls, adding apologetically: "I got very organised in my school days because my parents said that if I didn't finish my school work on time and I didn't keep my grades up, I wasn't allowed to go dancing and singing. But I think I'm a little OCD when it comes to things like dates and times."

Williams had been due to study interpretation and translation at Herriott Watt University in Edinburgh. "Doing Chitty made me grow up very quickly. Maybe I missed out on making those friendships that people make at university," she says. "But I don't think it was the right lifestyle for me – I don't think I would have fitted in. I don't drink. " She's in the second year of a six-year part-time Open University degree in Humanities – when term starts, she will have to fit in 16 hours of study around her West End performances. Is it a fallback plan? "No. I have five A-levels, I speak three or four languages. So I have quite a few fall-back plans already. It's more personal development, I don't think we can ever know enough."

In her spare time she reads a "disgusting" amount ("part of me thinks I should actually be reviewing books for a living") and dashes home to Yorkshire. She's been living in London for six years and is about to buy her first house, probably in the South-west ("It's a little greener, I'm a countryside girl..."). While she's happy to talk about her two cats – Melodie and Ellwood – she's much more coy on the subject of her partner ("it's very new..." she giggles). As for the future, she's still young enough to train at drama school or opera college, but every time she considers it, another job comes along. "If I did that, I might not have a career to come back to," she says simply. "So for as long as anyone out there will let me, I'm going to run with it."

Lunch break is over and downstairs in the draughty rehearsal room, " Atomic", given a Muzak makeover, is playing in the background of a party scene. As the cast perfect their positions and anxious crew members tap at their Apple Macs, Williams prowls the margins of the stage in a pair of sassy black ankle boots, waiting for her cue. Adding a lacy glove here and a chunky bangle there, a fake cigarette and a slouchy stroll, she slowly but surely transforms into her cooler alter-ego. She might just make a Susan yet.

'Desperately Seeking Susan', Novello Theatre, London WC2 (0870 950 0921), to 19 April