It is the one thing that gives energy and purpose, that can drive her to get up in the morning, rather than lie in bed for hours. Acting takes her mind off her condition, which is not something on which she likes to dwell.
"I think you can spend life full-time, researching, exploring and living in the world of your disease," she says. "I try not to. I try to live in another world." And most of the time she does. For instance, she rarely spends time at groups or centres with other sufferers of her condition. "It can be disconcerting and I can feel like I don't want to be part of that club."
"In a funny way," she adds, "I still behave as if the disease has nothing to do with me – it's another person. But then, of course, it's constantly with me. It's in everything I do. I can't avoid it."
In the warm days of early July, we meet at the Glue Factory in Glasgow, where she is rehearsing a new National Theatre of Scotland show, My Shrinking Life, about her condition, a disease that is particularly common in Scotland and has been described as our "hidden epidemic". The heat, lately, has been wearying. It brings on the fatigue, and accentuates the motor symptoms associated with multiple sclerosis, of which she has the progressive, not the remitting form, which means there is no getting better, only getting worse. However, in spite of her limitations, the actor wants to be physically challenged, and has told her director, Belgian Lies Pauwels, "Don't feel nervous about asking me to do too much, I'll tell you if I can't do it."
The show, in which she performs alongside dancers and also a young girl representing her youthful self, has been brewing for about six years. In that time it has gone through many different stages of development and been contemplated in several different forms: as a one-woman show, as something she would direct but not perform in, as a script-collaboration with Louise Welsh. "At one point," she says, "I thought, I can't be in it, because I can't do all the movements any more. But then I decided, no, I have to be in it and that's the point: that I can't do many of the physical things that I used to do, but I'm still the same person and I'm still an actress and director and I still have the same talent or experience that I had. It's just that I'm physically different."
My Shrinking Life is a devised piece of theatre, based on all the scripts and material that Peebles, 58, worked on in recent years and also a series of interviews, conducted by a researcher, with friends, family and medical professionals who have watched the journey of her disease. The show, she says, will not "just be autobiography". "I've done the documentary," she says, referring to Multiple, a frank TV programme she made in 2006 in which she came out as having the disease. Nor will it really only relate to her particular condition. "Most people have some kind of physical issue as they get older. Age is a factor and it does affect everybody."
In fact, the actress has produced a significant body of work, as a performer and director, both before and since her diagnosis. Her performance as Lady Macbeth, in Michael Boyd's celebrated 1993 production was one of her stand-out performances. And in the 12 years since she learned she had the disease, she has directed the lauded film AfterLife, appeared as Mother Courage in a touring production of Brecht's play and directed numerous theatrical productions. "But, at the same time," she notes, "there are a lot of people, working on the main stages, who have not cast me. Maybe they don't know how to use me. Or they're too afraid, they might go she's got a crutch or she's got a stick, how do we use that for that character?"
Meanwhile, there are ways in which her life does appear to be shrinking. In the past few years, since she was last interviewed by this newspaper, she has gone from being able to go out and walk around with a collapsible stick inside her bag to needing crutches while outside or, for particularly long distances, her mobility scooter. One of her worries about this show was that by the time rehearsals actually started she would not physically be able to perform. She is fearful, she says, that by this time next year she will be in a wheelchair. But she determined to hold that day off. "Some people with MS maybe have gone into wheelchairs a lot earlier and I've refused. I've refused to get a crutch until absolutely I have to because I think the more you can do the better it is."
Then there is her personal life. Peebles currently lives on her own, has no children – and never wanted any – and says it is more difficult to meet people, to form a new relationship. Others have encouraged her to do internet dating. "But," she says, "how would you even begin to do that when you're, firstly, not 35 years old and you, secondly, have some kind of condition or disability. And also I believe in chemistry. You meet someone, you're chatting and you get on with them."
Multiple sclerosis was a part of Peebles's life long before she was diagnosed with the disease. It was there, even in her own childhood. Her father, once a strong and very physical man, who played for Hibs, was gradually rendered immobile by the disease and died, at 56, when she was 17 years old. Sometimes, when she observes her own foot dropping, she sees in her mind her father's foot and the way it dragged. But, she says, until she was diagnosed, she had never dreamt that she would get her father's condition. When she told her older sister of her diagnosis, she recalls, "Sheila said, 'No no no, it's not that. You just think it is. I think I've got multiple sclerosis all the time because of dad." I said, 'Well I don't. I never thought it.'"
As much a part of the story of her relationship with MS is her mother, Marie, who passed away last year. Her death, through heart attack, came as a sudden shock, since she was still active and walking huge distances. It was, Marie, after all, who stoically raised her children, without fuss, as her husband's condition deteriorated. Peebles talks of how hard she found it to reveal her own diagnosis to her – indeed she did not for three years. "My mum was absolutely raging. She said, 'You should have told me first.' She was so upset by that. But I couldn't bear to tell her. I knew it would be so painful for her."
Peebles believes one of the "invisible" often unrecognised aspects of MS is the fatigue. "It's very difficult to explain to people unless they've experienced it themselves. I often think it's like marathon runners when you see them go all wobbly. And for me I always have a level of it." Last year she had a bad fall and was given steroids, which delivered a sudden oomph of energy, and she thought, "Wow, this is what I used to be like. A total buzz. This is normal life."
Even going to a show at the Edinburgh Fringe can be a monumental effort. Tam O'Shanter was one of the few she did make it to this year, but even the distance from the train station to the Assembly Halls, uphill, was beyond her usual walking range of about 200m, so she had to take a taxi. Then when she arrived at the venue there were stairs to scale. Like many of her stories, this is one that rolls out like comic farce. She recalls, for instance, at one friend's birthday dinner in a restaurant, a search for a disabled toilet that led her to a door behind a black curtain and a room where the light switch cord was so short it looked like someone had eaten it, and finally a loo with a cracked seat. Peebles laughs at all this. She does so frequently. It's one of the things she wants people to do when they come to see the show. Laugh.
My Shrinking Life is at the Tron Theatre, Glasgow, from September 8-15 and then visits St Andrews, Edinburgh and Inverness