The week before Christine Entwisle started rehearsals for the National Theatre of Scotland's revival of the 2004 Edinburgh Festival hit The Wonderful World of Dissocia, she was stopped at her local market in Cumbria by a tiny old woman with a bad leg. After she'd talked about how she'd fallen, and enthused about how she went to Blackpool every year, the woman let slip how, when she was younger, her mother wouldn't let her out of the house. Even worse, she'd been told for years that she was brain-damaged. She only ventured over the doorstep once her mother died.

Entwisle would love to write the woman's story - which could have stepped out of Dissocia's dark lysergic fantasia - but isn't sure if that would be exploitative. For now she's given her new friend a box of chocolates, and plans to send her a postcard from every town Dissocia tours to.

As well as illustrating her curiosity and compassion, the incident points up Entwisle's creativity, channelled via a variety of means. Her comedy duo, WonderHorse, are regulars on London's underground cabaret circuit. There's a sound installation she was commissioned to make in a Cumbrian graveyard, and a feature-film script about a disaffected goth called Trevor. There are plays, too, and short films. One, about a woman who becomes obsessed with Michael Parkinson, is already shot. In the evenings after her rehearsals for Dissocia, Entwisle cycles home to edit it.

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Such initiatives are a million miles away from her perceived lot as a jobbing actress - although perceptions have changed substantially since she first took the role of Lisa in The Wonderful World of Dissocia three years ago. Anthony Neilson's play tells the story of a woman's uproarious, ultimately self-destructive voyage into her troubled imagination; Entwisle's unfettered display of joie de vivre and vulnerability won her a Bank of Scotland Herald Angel Award at the 2004 Festival.

It says a lot about the fickle fortunes of the profession that she hasn't been offered an acting job since. It would be enough, one suspects, to drive a woman mad. "It's a very bizarre existential experience," says Entwisle, dryly rationalising her return to a role where she's put through an emotional wringer.

As is usual with Neilson's work, Dissocia was written in the rehearsal room, with actors building up their parts as they went along. Entwisle's involvement can be dated back to a meeting with Neilson, whom she already knew, at the wake for their mutual friend Jeff Nuttall, poet, artist, actor and one of the English avant-garde's unsung heroes. (Nuttall had appeared alongside Entwisle in a radio play of Neilson's; Entwisle had worked with The People Show, the theatre collective Nuttall co-founded in the 1960s.) Neilson told Entwisle he was considering her for his new project. "I remember standing on the steps trying not to bite Anthony's hand off," she recalls. "He said he'd maybe give me a ring, but that, you know, it probably wasn't up my street."

Until the week before rehearsals were due to start, Entwisle heard nothing. But she's used to Neilson's peccadilloes, and has been ever since they bumped into each other in Cardiff while working on separate projects more than 15 years ago. In 1994, she recalls, she enlisted his services as director for a self-penned solo Edinburgh Fringe show. Despite a "not entirely harmonious" experience, it was The Woman in the Attic's troubled protagonist, Entwisle suggests, that Neilson might have remembered when casting Dissocia. "He knows I can do mad," she deadpans, "so that probably helps."

It was 2002 before the pair worked together again, on Edward Gant's Amazing Feats of Loneliness. "That was the fire and brimstone time," recalls Entwisle. There were shouting matches, and at one point she stormed out of rehearsals. "Basically it's because we really love and respect each other," she says, "and occasionally it kicked off. But me and Anthony have had all our bust-ups already. We disagree about lots of stuff, because we come from very different theatre backgrounds, but we share a sense of the ridiculous and a very dark humour, and that helps. Having done a play with him once, it's never as bad as that again. She laughs. "Some people say the same about childbirth."

Weaned, in her own words, "on telly", Entwisle wanted to act from an early age. With her grandfather a miner and her mother and father working in the offices of the Sellafield nuclear power plant, Entwisle and her sister were the first generation with any opportunity for higher education, and her family had her future mapped out: doctor or maybe solicitor. For a shy girl not always fond of herself, however, the prospect of becoming someone else held far more appeal.

"I inherited from my mother this tendency to emote with films," she says. "We used to watch afternoon matinees, and she'd cry at the sad bits and do all the faces. I liked doing accents and mimicking people, and ended up doing the same thing." Still, Entwisle turned down her first stage role - Buttons in a Sunday-school panto - in favour of a less attention-seeking part as a sprig of mistletoe, a piece of magical-realist casting that could easily have made an appearance in Dissocia.

Rather than drama school, Entwisle took a performance arts degree. Encouraged to make her own work, at the age of 22 she put on her debut show in London. Slipper Trips, a grown-up sequel to Cinderella, set a benchmark, and Entwisle went on to work with The People Show and the Polish-influenced female-driven company Scarlet Theatre. She insists she didn't go out of her way to chase left-field work, but admits that "it's home". Her sole experience of a commercial west-end show was "so dull I forgot to come on twice. I was in the green room talking about hairdressing because it was more interesting than what was going on onstage. Honestly, I couldn't wait to be shot of it and run back into the comforting arms of experimental theatre."

While her only other brush with the mainstream was a part in Attachments, the short-lived TV drama she describes as "a dot-com version of This Life", Entwisle says she's "crucially and fatefully ambitious." She's justifiably proud of Lisa, although "obviously, all the characters I play are bit cracked". Despite, or possibly because of this, Entwisle "adores" comedy. Hence WonderHorse, where making theatre is as much about having fun as it is about causing trouble. She's the first to admit, however, that she's unable to settle on any one thing. Another mooted project is The Old Ones Are the Best, a series of videos of old people telling their favourite joke.

"Old people in homes just sit there and watch the telly," she observes, "and it's such a travesty. But I'd bet they'd love someone to go in there with a camera, spend half a day talking to them, and just shine a light on them."

Entwisle says she wants to be "not liked, but listened to; taken seriously". Her heroine is the American performance artist and film-maker Miranda July, whose film Me and You and Everyone We Know sums up for Entwisle the loneliness of the long-distance actress. "She's really in charge of her own material, which sounds incredibly egotistical - but when you do theatre, there are no footsteps, whereas if you film something or write something, you've got this thing, you've made footsteps, and you can prove it was there. With Dissocia the first time, it's as if the sand blew over it and left nothing."

If the mainstream stage did come calling, though, what would be her ideal part?

"Lady Macbeth," Entwisle blurts out, like it's something she's been waiting to be asked for ever. Casting directors take note. It would be mad not to.

The Wonderful World of Dissocia previews at the Tron Theatre, Glasgow, on Wednesday and runs there to March 10 before touring.