Donna Franceschild never meant to write Takin' Over the Asylum, her hit 1994 TV comedy drama set in a mental hospital.

Neither could the American-born playwright have predicted that the six-part series would provide high profile break-out roles for its two stars, Ken Stott and a young David Tennant.

Franceschild's new stage version, which updates the action to the present day, already has a head start, with Iain Robertson stepping into Stott's shoes as double glazing salesman turned hospital DJ, Eddie Campbell, the patient originally played by Tennant, looks set to be given extra edge by Brian Vernel, a second-year acting student at Glasgow's Royal Conservatoire making his professional debut following a remarkable performance in a college production as Macbeth.

"He's pretty special," Franceschild says in hushed tones on a lunch-break from rehearsals of Mark Thomson's co-production between the Royal Lyceum, Edinburgh, and the Citizens Theatre, Glasgow, "but they're all amazing. Everything tells you Iain Robertson shouldn't be right for this, but he just walked in and was. He's just a phenomenal actor. If you see the things he does, he just has this way of being so centred and real.

"It was written for someone in their early thirties, but Ken Stott was so fantastic we had to have him. We're very lucky. With the original Asylum, we felt, what are the odds that you could put together a cast where there wasn't a weak link? I feel the same about this one. Even the people who are playing what we call the second line loonies are amazing to watch."

Franceschild was first approached to transform Takin' Over the Asylum into a stage play several years ago for a production that never worked out. She describes the current production as coming "out of the blue. My agent knew, but I think wanted to spare me the disappointment of it not happening again."

The current production will also reunite Franceschild with Royal Lyceum artistic director Mark Thomson. The pair first worked together on a profit-share production of an early play, The Sunshine Café, at the Etcetra Theatre in London back in 1989. They also led some workshops for young writers at the Cockpit Theatre around the same time.

"He was very good," she says, "even though he must have only been about 21. I'm a great predictor, and I said the same thing about David Tennant, and I think we can agree I was right on that one too."

Whatever other predictions she makes, Franceschild stresses that her stage play will categorically not be a cut-and-paste version of the TV series. "I didn't want to go back to it unless I could reinvent it," she says, "so the first thing was to set it now, and the second was to set it all in the hospital. I also had to lose some of my favourite little plot-lines, but that was okay, because you have to learn to kill your babies. I decided not to read the script of the original or watch it, because I thought the things that are important and the things that I need are going to stay in my mind, so I thought of the original as source material.

"Because of the passage of time, that was relatively straightforward, so then I had to make it work for the stage, because the TV version was episodic. Some things changed in a really interesting way, because I'm coming at it in a different way. Mental health as a topic has moved on, but the stigma's still there. That was important, but the landscape has changed."

In between her two versions of Takin' Over the Asylum, mental health issues have become a major part of Franceschild's life. While this undoubtedly comes in part from her own experience of bi-polar disorders, she now describes herself as a campaigner.

"One of the issues that was maybe lower down in the mix in the original and is much higher in the mix now is the issue of employment," she says. "That's a huge hot-button issue now among mental health activists, of which I count myself one. It's this idea that, theoretrically, they can't discriminate against you because you've got mental health problems, but in actual fact, of course they do. All they need to do is say there was a better qualified candidate in the way that's happened for years with black people or women.

"Unless there's a pattern, it's very difficult to prove that this is going on, so it's a big issue now of the whole thing about Information Disclosure. If you disclose, you might not get the job. If you don't disclose, you could be in the job and, say for example, have an episode of depression, and that will end up being a sackable offence because you didn't declare you suffered from depression.

"That's a long way from celebrities saying they have mental health issues, which I don't criticise, but celebrities will probably be alright. Somebody trying to get a job after being hospitalised with mental health issues will probably find it much more difficult. I suppose what I hope the effect of this play will be on some of the audience, anyway, is thinking, not in terms of celebrities, but of people really struggling at the sharp end."

Franceschild first came to prominence when an early stage play, And The Cow Jumped Over The Moon, was picked up by BBC TV's The Play on One strand. The play featured a hospital DJ, which Franceschild joked to director David Blair could be its own sit-com. Blair took her seriously, eventually commissioning her to write what became Takin' Over the Asylum.

SHE recalls: "I didn't want to do any more hospital drama,. "But most drama in one way or another is about being in crisis, and people in mental hospitals are, by nature of being there, in crisis. So I stumbled into it, and along the way became an activist, and I will do anything to help de-stigmatise mental health issues. It's much easier to say you've got depression now, and I think we've moved away from the idea that it's a sign of weakness, but something like schizophrenia, we haven't even touched yet."

Takin' Over The Asylum, Citizens Theatre, Glasgow, February 14-March 9; Royal Lyceum Theatre, Edinburgh, March 13-April 6.

www.citz.co.uk; www.lyceum.org.uk