'I always have this dream that I'm going to write something very soft and happy, and it never seems to happen," says screen and stage writer Rona Munro. She certainly hasn't lived her dream with Iron, a play set in a grim women's prison where a murderess is reunited with her estranged daughter - the child of the man she killed. "Rarer than a unicorn" is how Munro's antiheroine describes herself; of the 70,000 people imprisoned in Britain today, 5,000 are lifers, but only 165 of those lifers are women.
When researching the play, which transfers this week from Edinburgh to London, Munro was struck by "the ordinariness of it. Lifers tend to have a prison routine. So when you visit, you're going into someone's room. It's like a bedsit. It's not like going into an iron-bars environment. The kind of conversations we had were very, very ordinary. We'd never talk about what they had done."
She was fascinated by "trying to get your head around what makes someone kill", and while she dramatises our curiosity about what drives people to commit crimes, she approached the subject in a very personal way. "Rather than thinking what made her kill, I was thinking about what would make me kill. With some women, the morality of it is very clear-cut - for instance, with women who have been defending themselves against abuse and who still end up inside, but I was more interested in the morally ambiguous cases. And I wondered: was it 'There but for the grace of God'? Could anyone do this, in the right circumstances? Or is it a particular type of person? And what does locking them up for life achieve? What do we want prisons to be? Is it punishment or rehabilitation?"
The play doesn't answer those questions; it is resolutely unpolemical. Unlike her director, Roxana Silbert, Munro has never done any drama in prisons, and has no plans to show Iron to prisoners. "Can you imagine?" She shivers. "That would be brutal." This puts Iron in a very different context from other plays about imprisonment such as Tanika Gupta's Inside Out, commissioned by the women prisoners' and ex-offenders' company, Clean Break.
But Munro's research did make her think hard about the morality of locking people up. One character, a prison guard, questions his own function within the criminal justice system, and that, says Munro, "is purely me".
It is typical of her work that the authorial voice comes through in such an oblique way. She bucks the trend for female playwrights to centre their stories on autobiography. Instead she spends a lot of time researching - although not usually in a library. Bold Girls, her breakthrough play, about women gossiping and bickering around a kitchen table in Ireland, was "based on chats I had with women at two in the morning over a bottle of gin". And for her new TV film, Rehab, directed by Antonia Bird, she stayed in a drug rehabilitation centre near Oxford. Not a fancy one like the Priory, but one where most of the patients had come from prisons or off the streets; "a very tough environment".
She "lives with" her characters while she is writing them, and can sometimes get almost too close to them. "My boyfriend went through quite a rough time when I was writing Iron. There were bits of it that were quite dark." But perhaps she couldn't write such warm and powerful drama if she wasn't able to lose herself in it. "For me, it's about empathising with the characters. If I can't imagine their reality from their point of view, I shouldn't be doing it."
Now 43, she started writing plays at school in Aberdeen and decided she wanted to be a writer when she was only 11. After university she worked as a cleaner and wrote in her spare time. Following various plays on the Edinburgh fringe, the Traverse gave her her first commission in 1983: the result was the elliptical Fugue, about a woman who claims to have seen a ghost. She went on to win a string of awards for such plays as Bold Girls, Snake and The Maidenstone (written partly in the Aberdonian dialect she grew up with, which, she says, "was an act of madness that probably doomed it to obscurity"). She also writes regularly for MsFits, the feminist theatre company she set up with the actress Fiona Knowles.
"We were going to self-defence classes together and we were crap at it - we would tentatively tug at each other, and one time I ripped her blouse - so we stopped, and we started going to the pub instead. We were talking about the kind of theatre we wanted to do and so we just decided to start doing it."
At first they performed sketches and songs as a double-act, but now Munro writes a one-woman show every year, which Knowles tours all round the UK. Last year's, Stick Granny on the Roofrack, was "about how you want to kill your own family. Basically," Munro laughs, "it was Iron - but lighthearted." She has just finished this year's, Women on the Verge of a T-Junction, about three women who crash their cars.
Her MsFits plays have a definite message of empowerment, where "the agenda is still to put on theatre for women about women". In her other work, the characters come first, and she has long given up performing and doing community theatre, saying, with a laugh, "Fiona takes care of all that. My social conscience is on the road."
But she would still describe herself as a feminist playwright. "I've been doing this 20 years and we're still having this conversation; it's depressing. But at the same time, there aren't enough women's voices out there."
The women she writes tend to be powerful ones, particularly in her films. Her first screenplay, in 1994, was for Ken Loach's Ladybird, Ladybird, based on the true story of a mother at the end of her tether. Her second, a commission from German director Max Färberbäck, has become something of a cult. Aimée & Jaguar charts the unlikely romance between Lilly Wust, a bland, blonde German hausfrau, and Felice Schragenheim, a glamorous Jewess, who fell in love in 1940s Berlin and were torn apart by the second world war.
Here again, her raw material was people's real, traumatic lives, but her take on them was far from remote or exploitative. "In the process of writing the script, I forgot that they were real, and when I saw it, I suddenly remembered that Felice was a real person, and all those things I was writing about, she really did them, and now all these people have seen her, and I felt she'd have loved that. I felt it was hers."
Although Iron is beginning its London run and Rehab is awaiting a transmission date, Munro is already looking for her next project, and hoping that maybe this will be the one that isn't gritty or dark or shocking. But whatever the subject matter, she can't imagine doing anything else. "Even if I stopped being paid to write, they couldn't stop me doing it. As long as I can type. And anyway, I can't do anything else. Except cooking."
· Iron is showing at the Royal Court Theatre, London SW1. Box office: 020-7565 5000.
