SHE LOOKS like an older version of a younger Sporty Spice. Dressed in a white vest and black stretch trousers, Karen Dunbar is sitting in her King's Theatre dressing room, plucking segments from a lunchtime tangerine. Her bare arms have that planed definition that takes hard gym-graft for a woman in her mid-30s. Dunbar is 36. The dolphin tattooed on her right shoulder adds to the impression of body awareness.

At the recent Scottish Bafta awards, she was photographed wearing a satiny plunge-necked dress. The only thing missing from her straight-haired supermodel look was a practised paparazzi red carpet pose: Dunbar, smiling broadly with her hands grasped school-photo style in front of her stomach, didn't look like a veteran. One tabloid captioned the picture: Eschewin' The Fat.

At last year's Baftas, Dunbar was all pin-striped suit and trainers. So what's behind the image-change? "I think," she says, "that I've probably grown up. I'm a late developer and part of me is just a big kid. I'm glad of it because it fits in perfectly for this pantomime role."

She has, she confirms, lost "a lot of weight" this year. "And I have done that intentionally. It wasn't so much that I thought, Oh, I'm going to lose this amount', it was more the result of a change of diet: my response to the kind of food hangover you get eating stuff late at night."

Energy levels dwindling, she would wake in the morning feeling lethargic. "Bowel movements," she says, "are something you have to think about at this time of life." Losing weight was secondary to feeling better. She joined a gym near her Glasgow west end home, and regularly coaxed herself into attending. "I'll make a deal with you," she'd tell herself, when she contemplated ducking out of a workout. "Go and put your kit on then take it off and come back down the road. It'll take 15 minutes." Once at the gym, of course, she would stay.

Dunbar's size has always been difficult to determine. In Chewin' The Fat and her own Karen Dunbar Show, she has played both "roomy wummin" and slender, glamorous types. Once, she slipped into a pair of black leather trousers and did a send-up of Shakira's Whenever, titled Assita. "Look at this arse," she sang. "Bet you wish you had an arse like this." Watching, I remember thinking: "Nice buns."

Dunbar's willingness to portray herself as anything from glam to gross is part of her appeal. Although she is gay, she knows how to play heterosexual desire at its most stomach-churningly grotesque. I mention a sketch in which a young man arrives at a council house to be serially seduced by the daughter, mother, grandmother and finally great-grandmother of the family. The scene climaxes when Dunbar dodders out: great-granny in a Sex Bitch T-shirt, tongue lolling out of her mouth. "It is more important to me to be funny than to look good," she says, when I mention the scene. "Fortunately, because of the nature of the sketch shows, they can usually tart me up later on. And it's nice to have that before and after shot with three seconds in between."

Physically, Dunbar's trademark has always been those extraordinary nostrils. On her dressing table is a copy of the collected letters of Kenneth Williams: another comedian who knew how to work a nose. She describes him as "a big influence".

Today, a black woollen cap shadows her eyes, drawing attention to those flaring apertures, which for most of the interview stay calmly deflated. Our conversation has an acoustic accompaniment. From a speaker in the corner comes a slow, monotonous drum beat. It is Dunbar's audio connection to the stage, and it has, she says, to stay on.

We try to ignore it and talk about growing up. Why try to be grown-up now? "I don't know," she says. "There hasn't been anything life-changing. But I find from general conversations with women my own age, that this is how it is in your mid-30s. It's a time when most of us start to think, OK, I've got a general idea of what I'm doing and where I'd like to go. I'm comfortable enough. I don't have a castle in the Highlands and I don't need one. But I can talk to a plumber. I can take a pair of trousers back to a shop without going into a minor breakdown."

She recalls a younger, more nervous and neurotic self, who once set out to return something to a shop and had to have a cigarette before she could persuade herself to go in. "I remember vividly thinking, What is the matter with you?' Now I've outgrown that. I didn't have to go and workshop it or anything. It just worked its way out of the system."

She remembers a different Karen Dunbar: one who was less comfortable in her own skin, less willing to make mistakes or to allow herself, in everyday life, to "look like a tube". That Karen Dunbar verged on workaholism, filming the second series of Chewin' The Fat while maintaining her job as a barmaid and karaoke star at Glasgow's Bonkers bar. "I was 28 at the time," she recalls, "and working like that was enough to let me know that I don't like money that much, not enough to die young. We were filming sometimes at six in the morning, finishing at 9pm, then I'd be at work in the pub at 9pm and home at 1am. If I had kept that up, I don't think it would have mattered how healthy I was, how enthused. We are only wee bodies of energy, we only have so much in a 24-hour period."

Still, it's not hard to see why she took on such a workload. The girl from Ayr who had started out as YTS post office worker then found her ideal job as a karaoke performer, must have looked out on to the vast savannah of mainstream success and decided to cling to some financial security.

In conversation, Dunbar takes long duck-dives into seriousness. She describes herself as "a bit of a cock-eyed optimist". Lately, she says, she has been persuading herself not to let her mood be affected by the weather. "If you don't like it, Karen, move to the Sahara. This is Scotland."

She describes the moments during performances when she loses her self-consciousness. "Existentialism," she says, with only a hint of comic irony. "That's what this job is about. This is what the magic of it is. I'm standing backstage. My guts are churning. I'm running through my lines. I'm coming up through a trap door. I've got that massive dress on." She points to a huge glittery puff of a costume in the corner. "I'm holding on to my wand. All this is running through my head and they put me through the trap and I walk out in front of an audience and whoosh: my past and my future - everything - is wiped out." She pauses and adds: "That's why people pay a lot of money for hardcore drugs."

One on-stage experience stands out. "I was aware of all of it," she recalls. "Me on the stage, the audience, everything, and there's no way to evoke that or to say, Well I'm going to try to feel that tonight.' I was really high. But it was so fleeting. Afterwards I thought, That was weird, that was great, that's the point.' You can't stay at that point, though. You can't live life at that level. But, oh wow, to touch on it once and to know that it's there. That's enough."

Here in the dressing room, the drum beat from the speaker in the corner has got faster, the slow thud now a rattling beat, and it becomes harder to concentrate. Dunbar strains to keep to the point. Eventually she leaves the room promising to get the device switched off. When she returns, there is silence. "I waved my wand," she explains.

The wand she is describing is one of the props of her character in the King's Theatre pantomime, Sleeping Beauty. As Nanny Begood, Dunbar has followed in Elaine C Smith's steps and taken on panto's traditionally male comic role. The slimmed-down glamour puss has thrown herself into playing the classic big, brazen caricature of camp masculinity. Smith, artistic producer of the show, has given her plenty of advice. "Elaine said a great thing to me," says Dunbar. "If they believe it, you believe it."

Meanwhile, she has her own take on how pantomime works. "Without sounding too cheesy, the thing that brings the magic is the audience. That's the cherry on top of the cake and the cake doesn't really exist without the cherry."

As a child, Dunbar was a performer, singing at the local Labour club, or for passers-by from a pile of builders' sand. But neither she nor any of the other local Ayr kids went to the pantomime. "It's not part of my history," she says. "Our families weren't taking us. So most of the panto I know is from coming to the King's in my 20s and thinking, I could do that. Could I do that? I think I could do that.' When I was offered this role, I had to take it."

I ask about that dolphin tattoo. "That was by Terry," she says. "Do you know Terry the tattooist? He was kind of synonymous with Glasgow. Gone but not forgotten because he lives on on everybody's arm." She twists around to show the image better. "I got that 13 years ago. Wow, is it really that? It's funny at this age. I start hearing my mother again. The older you get, the quicker things go.' She was saying that when I was 11 and it seemed like the space of time between Christmas and my birthday in April was just the longest stretch. Now, bang, bang, they whoosh by. "

It's the one mention she makes of her mother, who died of a stroke when Dunbar was 21. Her family life is not something she talks about. There is a story to it. The couple who raised her, who she called mother and father, were not her biological parents but her grandparents, and the woman she called her sister was her mother, having become pregnant at 16. Dunbar is highly protective of her personal life. When I touch on her two-and-a-half year marriage to Strathclyde police officer Jennifer Fraser, she tells me: "I'll not be speaking about any of that today."

Has she any desire to have children? "Och, aye," she says, and adds, with a nod to an earlier reference to my recent maternity leave: "That's a big change isn't it? I remember one of my pals saying that so many of their opinions - their whole frame of reference - changed when they had kids."

Dunbar hasn't changed as much as it may physically appear. Though she admits to a weekly hairdresser habit, she still tends not to wear make-up. With the exception of a panda-eyed teenaged goth phase, she never has. "I've never really been too bothered. I know a lassie who has to get her make-up done before she goes out to the bin. I think, Oh, how could you?' But then I'm anti-that."

Although she acknowledges the "great feeling" that comes with weight loss, the boost to her self-esteem, this new look is neither life nor career-changing. It will not, for instance, influence the kind of part she plays. "I don't think, right, I've lost that weight, I can play more sexy roles, because I've already done that. Even at my heaviest I've played all those roles. I know I can do that anyway."

A glance through her back catalogue confirms that. What she plays is about something beyond the base matter of her physical make-up. It's the voice and the energy, her ability to tap in to the comic shorthand, that convinces. Auld slapper or shoeless drunk, we are carried along with Karen Dunbar for the laughs.

Sleeping Beauty is at the King's Theatre, Glasgow until January 12