Security guards reviewing after-hours CCTV from certain city-centre offices this winter would have had a bit of a surprise. If they were looking in the right place at the right time, they'd have seen a young cleaner, cloth in hand, occasionally breaking into a dance.

Debbie Findlay smiles broadly as she admits she occasionally practised her steps and lines for the Citizens' Theatre Community Company's production of They Shoot Horses, Don't They? while working morning and night as a cleaner in various locations. But for Debbie, 30, time was of the essence. When she first started rehearsing the April production, back in January, she was getting up at dawn to clean and going back to do another shift in the evening; in between times, she was trying to fit rehearsals around looking after her four children, aged seven, 11, 12 and 13.

For someone who, a year and a half earlier, had been in Cornton Vale prison, struggling with addictions to drugs and alcohol, it was a sea change, and one she credits with making her happier than she has felt at any time since her childhood.

Three weeks on from the final performance, Debbie still looks slightly stunned talking about it, as if she can't quite believe that it happened. "It's just amazing that at this time in my life I've had the chance to do something like this," she says.

It's afternoon and we're sitting at a table in the deserted lobby of the theatre. Debbie, blonde, model-like and glamorous, is quietly spoken, but punctuates certain points with a firm, direct gaze. "What I'd really like to say is that if I can do something like this then there are loads of women who can."

Engaging with the arts promotes confidence and positivity: statements like that are the stuff of Scottish Arts Council funding applications. Yet the dry report-ese is anything but when the evidence is sitting in front of you in the form of a poised, confident woman who's been through what Debbie has.

Debbie does not want to say why she went to prison. "An incident happened, I don't want to go into it, but the same thing could happen to you, it could happen to anybody," she says. "I was just a normal mum with four kids and something happened." While there, she had a drug problem. "I've had addiction issues," she says, "drug and alcohol addictions." She also describes being separated from her children while in prison as "the worst thing that could ever happen to a woman".

In late 2006, she saw posters in the prison advertising a Christmas musical production, Olivia Twist. There were going to be three weeks of rehearsals and then four shows, including one for families. Debbie, who had loved dancing and drama as a child, went along to the first meeting, where she met the director, Elly Goodman, a workshop specialist from the Citizens' Theatre. It was a pivotal decision.

"Elly just creates this feeling of positivity. She's like human Prozac," says Debbie. "I've seen her working with some of the toughest, hardest women in Cornton Vale and she did so well for them. It gave us a bit of light, positivity, and there isn't much for women in that situation. I didn't have a big role, but I can't put into words what Olivia Twist did for me." Instead, she quotes a fellow member of the cast: "It enables you not to be you for a time."

This experience, of trying on a happier persona, helped Debbie make up her mind that life had to change. She gave up the drugs, but that was only the start: to stay off them, she was going to have to make a clean break with everything - and everyone - connected to them."You're sick of that kind of life, because it's downhill constantly," she says with a shudder. "You can never keep a bit of that life."

When she came out of prison she started having counselling at the 218 project in Glasgow, a support unit working with women offenders run by the charity Turning Point. There, she heard about They Shoot Horses, Don't They?, staged by the Community Company. Now in its tenth year, the company puts on shows on the Citizens' main stage to the same professional standards as any other shows at the Citz, but using a cast of local non-professionals, whose numbers are increased by the inclusion of some former offenders and stable recovering drug-users. Her counsellor, Lorna, asked if she wanted to be involved. Did the prospect make her nervous? Not quite. "I jumped at the chance - I literally jumped up and down," says Debbie.

They Shoot Horses, Don't They?, set in Depression-era Hollywood, follows the fortunes of a group of contestants taking part in a dance marathon and vying for a $1000 prize; as in any modern reality TV show, it features a hotchpotch of characters: starlets, has-beens, the deluded and the desperate.

Despite lacking acting experience, Debbie was cast as one of the leads, Alice, an uptight aspiring actress and a very different character from Debbie herself, who was played by Susannah York in the 1970 film version. As Alice, Debbie was the picture of retro glamour in a long sequinned gown and Marilyn Monroe wig, an added triumph given that Debbie was herself once 18 stone, an admission that amazed some of her fellow cast members. "I said to them, You don't think I had four kids and came out looking like this!' I know that feeling, that you're ugly. I've had so many dark horrible times - and now I've done something like this."

The Herald's theatre critic, Neil Cooper, reviewing the show, described Debbie as "a vibrant, focused actress" who held her own with more experienced members of the cast.

Debbie's children gave her their wholehearted support throughout, and two of them came to see her perform. All in all, for Debbie, the show was a transformative experience. "The minute I made a positive choice about that, everything seemed to fall into place, even down to work. It was amazing and I'll always remember it."

For Elly Goodman, who has been doing theatre workshops and productions with vulnerable groups for 19 years, this is what makes it all worth while. She says that Debbie's skill in They Shoot Horses "blew my mind to kingdom come", under- lining the great advantage of drama, that no prior experience or education is required: "A keen, willing open mind: that's all you need."

In a prison setting, drama helps inmates build stronger bonds with one another and their self-esteem grows. "Someone's saying to them, You're doing something good', which is important because it's a really negative environment. People say it's a cushy lifestyle. In fact, it's a closed, horrific place."

Goodman has done four shows at Cornton Vale and each has worked in a similar way: a professional playwright works with prisoners to develop a script, putting in plenty of jail jokes, then Elly and a musical director come in and work with 25 women of all ages. "I make a real point of not wanting to know anything about their backgrounds," she says. "We just get on with it as if we're working with any other group."

Prison performances, though, are unique. "The audience are all shouting and screaming," she says. "A lot of prisoners take drugs but they always say that the buzz they get taking their bow, with the applause and adulation, is phenomenal. They grow 10ft tall."

The Community Company has been going for 10 years, with funding coming from the Glasgow Community Planning Partnership. Uncertainty is caused by the need to secure funding each year but Goodman is anxious that the work should continue. "We are absolutely committed; this work has to continue," says Goodman.

They have a great advocate in Debbie. "In the darkest times you feel so alone and that no-one in the world knows that feeling, nobody. If I can do it, then anybody can," she says. That is not quite true - not everyone would have been entrusted with such a major role; as Goodman says, Debbie has real ability. But all are welcome.

Most importantly, for a year and a half now, Debbie has been "really, really well", though she takes nothing for granted. "I'm learning who I am," she says. "I'm still in the early times of being Debbie and I do take it day by day. I have to stay humble because I can't see the future. I've just got to keep it together."

She would "definitely" get involved in another show. Finding the time is still a struggle, but now she has the confidence.

"If there was a drama class, I'd go to it now, whereas before I'd think I didn't fit in," she says.

"I'll be able to tell my grandkids I did this show. I loved it, totally loved it."