Is there anyone who can do turmoil better than Juliet Stevenson? Never has loss, in all its salty, mucus-encrusted, heart-wrenching, wailing glory, been rendered so vividly as in her big film breakthrough, 1991’s Truly Madly Deeply — in which she played the lover of Alan Rickman’s recently deceased cellist. On stage, in Death and the Maiden, for which she won an Olivier award, Stevenson was at it again — violated and vengeful, exacting comeuppance upon her military-junta rapist. In her most recent television outing, last year’s three-part crime drama Place of Execution, even her investigative journalist, unravelling the clues to a grisly, 40-year-old murder, could not be complete without the furrowed-brow concern of a woman juggling all that and an imploding home life.

In summing up Stevenson thus, I’m being both irreverent and highly impertinent, of course, fully worthy of a slap. Across 30 years, she has built a solid and varied résumé as one of Britain’s most recognisable thesps, star of stage, screen and, not forgetting, radio. Still, you have to admit, she does that angst thing rather well. Brilliantly, in fact.

Currently, Stevenson is getting raves for what might well be the role of her career, and you’d be right in guessing there’s torment afoot. In Duet for One, which has just transferred to the West End for a 12-week run, she plays a former virtuoso violinist, Stephanie Abrahams, debilitated by multiple sclerosis and wheelchair-bound. The play presents Abrahams in a series of sessions with her psychiatrist (Henry Goodman) as she confronts her unimagined future. Not the cheeriest of subjects, admittedly.

A dialogue-laden two-hander, 2½ hours long, it’s an intense piece of work. “I go out on stage and it feels as if there isn’t a single part of me that isn’t used up doing what I do,” Stevenson says. “I mean, head, heart, soul, fingers, sexuality, everything is employed. It isn’t every day you can do plays like this.” The role requires her body to register all the tremors and tics of MS, but she never lets this smother the character’s emotional life. It’s an undeniable tour de force from Stevenson, and from Goodman, her unflappable shrink. (“A phenomenal actor and a wonderful collaborator.”) Their bond grows as Abrahams’s condition deteriorates; her mental state, meanwhile, veers from bravado to despair to serene resolve.

On a sunny Saturday lunchtime, Stevenson breezes in through the stage door. She is a little late — “I’m quite scatty,” she concedes later — and is still cloaked in the Outside World, having spent a typical weekend morning managing the children’s activities. (She has a girl, 14, and boy, 8, with Hugh Brody, a British anthropologist, who zips off periodically to a teaching post in Vancouver.) Now she must enter that strange thespian limbo — the “pre-show dither”, she calls it — which will see her shed the happy Hausfrau and slip into something... well, a little more tortured.

In turquoise cotton top and jeans, a youthful-looking 52, and without an ounce of fat on her — heart-and-soul emoting clearly trumping the gym — she leads you to her dressing room. There, you squeeze into the regulation insalubrious, tatty broom cupboard. Stevenson perches on the single bed, the one on which she attempts to get her head down for a nap between performances on a matinée day. The street noise of a lively Covent Garden beyond the cell-like window could hardly be less conducive to the transformative process, but she can isolate her thoughts. “I do compartmentalise, because I have to,” she states. “I’ve spent the morning organising our days — food arrangements, childcare and stuff — and I’m normally doing that all the way into the theatre.” In the old days, work was the be-all and end-all. Not any more.

Written by Tom Kempinski and first performed in 1980 (starring his then wife, Frances de la Tour), Duet for One is loosely modelled on the plight of Jacqueline du Pré. (Tragic cellists are a recurring theme with Stevenson.) It was made into a film in 1986, with Julie Andrews and Max von Sydow. “I think the important thing to say is, it isn’t really a play about Jacqueline du Pré, and it never was,” Stevenson asserts. “What Tom Kempinski was saying is that he was inspired by the circumstances of her story. She’s this prodigy who had this glittering career, married glamorously and then was struck by this devastating illness at a very young age. The woman I play is not like that. Kempinski would say he was writing himself.”

Neither is it a play specifically about MS, although she says she has “had some very moving letters from sufferers”. Rather it’s about someone denied the very thing that has been their raison d’être. “Many people say, with the illness, that the prime experience is about handling loss.”

That chair, though, is something else. An electric number, it is used in lighter moments to buzz round the stage like a fairground dodgem, relief for an otherwise static production. “We went to a wheelchair shop,” she recounts. “I got the guy to soup it up. I tried it, I went off down the street. It was somewhere in Wandsworth. Because Stephanie’s rich, she would have a quality one, but also I wanted it to be quite fast, to have a physical dynamic. The chair’s almost got to be like a personality.”

Stevenson has been a national treasure for some time. She didn’t go to university, she points out, but got into Rada, then slogged her way up through the usual rounds of spear-carrying and suchlike with the RSC, making her professional debut in 1979. Born in Essex, she grew up the daughter of an army officer and, like a lot of actors, had a peripatetic childhood, living in England, Germany, Malta and Australia. It was not the need to blend in that nurtured her thespian side, rather her capacity to cope with a transient way of life. “When I’d arrive in a new country, I didn’t have any friends for a while. I was quite self-sufficient. I don’t know what the connection is with acting, but I can set up home very fast.”

As a struggling actor, she lived in crappy flats and indulged in boho-leftie politics. On YouTube, there’s a 1992 clip of a party political broadcast on behalf of the Labour party, still yet to acquire its “new” prefix, in which she and Stephen Fry and Lord Dickie and Ben Elton and various hair-gelled luvvies implore the nation to shed the Tory yoke. She doesn’t want to dwell on politics. “Like many people, I was energised by the idea of a Labour government. What can I say? Like many people, I became fantastically disappointed with that Labour government.” In 1999, she accepted a CBE, but shunned the Buck House presentation. Of late, she has highlighted the plight of asylum-seekers. Last year, she took part in the short-film TV series 10 Days to War, playing the lawyer Elizabeth Wilmshurst, who questioned the legality of the Iraq invasion. “I think the way we were lied to over the war is an absolute disgrace,” she mutters. The series has just come out on DVD.

In the 1980s, Stevenson advanced beyond classical roles and excelled in plays such as Les Liaisons dangereuses, Yerma and Burn This, with John Malkovich. Then came Truly Madly Deeply, a film she reflects on with sadness, given that it was written especially for her by the late Anthony Minghella. “I don’t really like watching that glamorised version of grief — a single tear rolls down a perfect cheek,” she explains. “My experience of loss is that you feel trashed. Loss is not a glamorous thing.”

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