If there is one actress, however, who has earned the right not to abide by the rules, it is Hunter. Over the past decade and a half, she has zigzagged her way to the top of the profession with a series of luminous transformations. Since her first professional role, as a werewolf, she has been, among other things, an overweight mafioso, a pantomime monkey, sundry little old ladies and diverse Shakespearean ancients, most controversially King Lear. She can do monsters - she won an Olivier Award in 1991 as the bitter, crutch-wielding millionairess, Clara Zachanassian, in Theatre de Complicite's production of Durrenmatt's The Visit. She can also do children - Mamilius in The Winter's Tale, for instance. She can even do both at once, playing the eponymous shapechanger in Caryl Churchill's The Skriker. "Protean" is an adjective that stalks her in reviews.
Despite the fact that she's continuing her own chameleonic tradition, Hunter's latest metamorphosis is hardly child's play. Spoonface became a phenomenon from the moment she was first heard on Radio 4, in January 1997, with 11-year-old Becky Simpson voicing the part. How could she not be? Born "backwards", with a face "as round as a spoon", and dying of cancer to boot, the opera-loving youngster expressed a touching faith in the ineffable beauty and meaning of existence with artless articulacy.
The public response to the play is a story in itself. Hundreds of callers demanded an immediate repeat, and anecdotes spread about lorry drivers pulling over on motorway hard shoulders to have a good weep. The play's afterlife has been distinguished by awards, 35,000 cassette sales and a TV adaptation.
Just looking at Kathryn Hunter close-up goes some way towards dispelling doubts about the wisdom of the project. She can so dominate a stage that you forget how small and delicate she is, how child-like. Yet this five-foot wisp of a woman radiates strength, her intense brown eyes shedding years off her crinkly parchment-map of a face.
"It is fantastically difficult," she admits, her voice husky and hushed, as she raids a packet of Silk Cut, and lets out a breathy giggle, the first of many. "You're never going to be the character. All you can do is suggest her as powerfully as possible. You have to trust that the audience will want to go with you. It's an experiment, prompted by my gut-reaction to the text. I didn't think of it as a great acting challenge. I just thought, `Oh, that's interesting'."
She was unaware of the piece until Hall gave it to her to read after their collaboration on Brecht's Mr Puntila and His Man Matti, last year's soaraway success for the Almeida and the Right Size, which he translated and she directed. She couldn't have spotted the 33-year-old Newcastle writer at a better time - Hall is now snowed under with film and theatre commissions; his larky version of Goldoni's farce, A Servant of Two Masters, opens at Stratford next week. They're meeting up again next year, when she stars in another Brecht of his - Mother Courage for Shared Experience.
After two weeks of workshops at the Royal National Theatre Studio, and a run-through that apparently had Trevor Nunn in tears, Hunter decided "there was some juice in the idea". One early decision was to retain the feel of an interior monologue. Spoonface wouldn't speak to the audience, she'd be overheard talking to herself. Hunter went to several schools for autistic children for research.
"You see a lot of what they call stereotypic behaviour," she says, suddenly holding a hand in front of her face, absent-mindedly waggling her fingers. "I'm not doing `ill child', that seemed the wrong path to go down. The task was to find a lightness. The vocal register has to be slightly lifted, which brings out a comical quality in the lines." Her partner and the show's director, Marcello Magni, a Complicite co-founder, got her to stick on a red nose during rehearsals and clown around.
The emotion, the sad stuff, will be supplied by the audience, she thinks, as they hear the words ("a seamless panning out from the domestic to the cosmic"). She was moved most by the section in which Spoonface provides her own account of the Hasidic theology supplied her by her doctor. "When the world was made, God made it out of magic sparks... and all the magic sparks went into things ...and the whole point of being alive - the whole point of living is to find the spark". When she tells you about her past, you understand why it's become a credo for her.
The daughter of a Greek shipping magnate, she had a fairly privileged London childhood, but was far from carefree. During her teens, when she was still called Ekaterini Hadjipateras, she became severely anorexic. At the age of 21, in her final year at RADA, during a period of deep depression, she hurled herself from a window. Given the injuries she sustained, the doctors said she'd never walk again. She did, though, and the pain eventually subsided. But she still bears the scars.
Her survival was a watershed. "I thought, the point is not that things are not as you wish them, the point is to struggle. I changed my outlook - when things get difficult, I think `Right, here we go, let's see if we can move through this'."
She only got on her feet again properly after graduating from Rada, with a helping hand from the two biggest influences on her career: Chattie Saloman, who taught and directed her for the community theatre group Common Stock, and the members of Theatre de Complicite, with whom she has now done six productions. Rather than being excused from strenuous physical activity, she was encouraged to use what she had. "There was no ruminating about what I could and couldn't do."
Lee Hall says: "Her kind of bravery is energising." He ads: "She's totally unafraid as an actress. There's a passion that drives her. She doesn't stint on the emotion and the humour. Both of those aspects are incredibly important to me."
She sees the lighter side of the bleakest things. She has to, she says, "particularly after the suicide bid. There's something quite funny about botching a suicide. You see how ridiculous you are." Her landmark performances to date all have a distinctive quality of the tragicomic about them. Her tiny frame, bursting with feeling, conjures both beetling human insignificance and the obstinate, even childish, determination to go on - whether it's Clara Zachanassian, hellbent on revenge, or Galactia, the unflinchingly graphic war artist who takes on the Venetian State in Howard Barker's Scenes from an Execution, revived this year.
Derided by some critics for her portrayal of King Lear, she remains undeterred. "I want to do it again, several times before I go. I love Lear because he went off into the storm. He didn't say, `Oh well, what the hell, give me my slippers, I'll sit by the fire'. He went `Fuck you, I'm going out there'. I love that." She giggles again. As Spoonface might have said, this nice lady, she's got the sparks.
`Spoonface Steinberg' is at the Crucible Studio, Sheffield (0114-249 6000) to 18 Dec; and the New Ambassadors Theatre, London (0171-836 6111) 4-22 Jan