When film-maker Coky Giedroyc met a group of young prostitutes in Manchester, she immediately wanted to tell their heart-breaking stories. But the road to celluloid wasn't straightforward

"What was really, really scary was just the size of the screen," remembers Coky Giedroyc of her first foray into direction. "Am I going to be saying something big enough and important enough to fill that huge space?" As she'll happily admit, Giedroyc has had plenty of time to consider the question. While sister Mel graduated from stand-up to sofa-sitting in C4's Light Lunch, Coky was slogging for years on the other side of the camera. Working the festival circuit, applying to arts organisations, and shooting commercial quickies, she became, she says, "queen of shorts" before getting her first shot at a feature.

The result is Stella Does Tricks (right), the tale of a homeless girl trying to quit her London life of prostitution, while dealing with raw memories of a Glasgow childhood. The idea for the film came, Giedroyc explains, from a documentary series she made for Channel 4 some years ago. "In the course of researching the series, I spent a lot of time with a bunch of young girls in the North. I loved their company, but was heart-broken and amazed by their stories. When the series finished, I knew I wanted to turn that experience into something more."

Having met and become friends with the girls, Giedroyc was determined not to present her prostitute as a two-dimensional victim. "Sure, Stella's a victim of circumstance, but there's so much more to her. I wanted her to be a female scally with a wicked sense of humour and a rich emotional life."

Such a concept seemed alien to the screenwriters she commissioned to draft scripts. "Writers seemed to think `ah ... young prostitute. Drugs! Issue drama'. Or, make her into an epic, literary figure. The whore with a heart of gold. What no one seemed to understand that what was really important were her aspirations. She had to be someone worth more than life had given her."

Luckily, while ploughing her way through a pile of picaresque preconceptions, Giedroyc began reading a collection of stories by AL Kennedy, and decided this "dark, humorous, strange" writing was exactly what the film needed. The pair met and "clicked", and Giedroyc's hunch seemed vindicated when Kennedy turned in a "blinding" first draft.

But their creative partnership was not without problems. "Alison's a novelist, so she found working with others difficult, and I'm a complete collaborator, so we ducked and dived around each other a bit. Looking back, the things we clashed on were quite technical. The first was whether Stella's childhood should be set in Manchester or Glasgow. I felt very close to the girls that I'd met in Manchester, and always dreamed it would be there, but Alison put her foot down and said it had to be Glasgow. There were some white lips about that one, I can tell you, but in the end I realised that I had to let go of my experience, and hand it to Alison to make it into fiction."

With a screenwriter in place, Giedroyc's next challenge was to find Stella. Surfing the hype from Trainspotting, Kelly Macdonald read for the part, then turned up again. And again. "She kept phoning back," remembers Giedroyc, "and saying she wanted to do it better." Still, she'd only been in three scenes in Trainspotting ("all I did was bonk Ewan," confessed Macdonald), and had no formal training, so Giedroyc continued to audition. "I screen-tested other girls who are much more technically accomplished actors, but I'd get bored by their performances. Kelly had no gimmicks up her sleeve. Instead, she was very internal, and had a stillness about her that was compulsive."

Stella's avuncular pimp, Mr Peters, was approached in a similarly oblique fashion. "I had a hunch that if I got the audience to feel comfortable with a culturally cosy, sitcom type, then their fear, when it came, would be much deeper," explains Giedroyc. So who is the perfect pimp?

Martin Clunes behaving nastily? David Jason getting a bit of extra nicker from the dollies? "David Jason?" asks a shocked Giedroyc, "he's much too sweet". No, the likely lad is James Bolam, who, she argues, has just the right blend of charm and menace. "From my experience, Alison's description of Mr Peters as a rather avuncular, well-spoken man who wears Pringle sweaters, plays golf and drives a Ford Mondeo is very true. I've met men like that who have this incredibly respectable persona, but who are doing deeply unpleasant things."

After Carine Adler's Under the Skin last year, it seems almost as though every new woman film-maker is compelled to make a film dealing with female sex or sexuality. Does Giedroyc feel she is following a pattern? "I'm quite evangelical about stories with strong female leads," admits the director. "And, by definition, most of those stories written by women about women are going to be in that territory. But in a weird way, Stella's sexuality remains incredibly private. What she's giving out has absolutely nothing to do with what she feels or cares about. That's why you never see anything. There's no voyeurism. No titillation. No sex. Stella's not interested in sex. Men are rather tiresome to her. That was so true of the girls I met up in Manchester. They were like girl-women. They didn't have a developed sexuality because it had been robbed from them early on. Their real life was elsewhere. For Stella it's about having dreams, about her aspirations to do something better."

Giedroyc concedes, however, that now she has got this story out of her system, it may be time for her to move on. "It is true in a way, that you have to get through all this to get on to other stuff - more unusual narratives. Women film-makers need to mature and be able to tell more stories. I want to do a comedy with five female friends for my next piece. It's about bloody time."

`Stella Does Tricks' is on release