And she's not bad at tennis, either

"With each performance she walks off the cliff just like in a cartoon. She doesn't look down and that's what keeps her in the air," says the director Stephen Daldry, mid-flow in an eloquent gush about Fiona Shaw. "She's fantastic to work with. She has an amazing, fearless connection not just with the role but the whole play. She makes the possibilities infinite. She leads a company marvellously, galvanising, accepting and never blocking. Working with her is obsessive. When you've done it once, you want to do it again."

Two seconds later, Daldry calls again. "Two more important things: I'm madly in love with her and there's no one I would prefer to spend the rest of my life with." Actually, Daldry's heart can be accounted for elsewhere, but if his was a lone rapturous voice, you might dismiss it as a dazzled luvvie's love-in. Far from it. Challenge anyone who's ever worked with Shaw and you find similar adulation and hyperbole.

Ciaran Hinds, a fellow Irish actor who played her lover in Machinal and her brother in the television Persuasion, says: "She has a magnificent intellect. Her approach is so fresh, so dangerous, you never know where you'll end up." According to her friend and most frequent collaborator, the director Deborah Warner, "What she does is beyond acting. She's like an eagle taking flight. A soaring thing. And it happens at a level that changes people's lives." Peter Wood recognised her genius very early and cast her, an unknown straight out of Rada, in The Rivals in 1983. "Her talent was a God-given thing. She's a very brilliant comedienne. There was no division between the text, character and herself. In a very artificial comedy she managed the rare thing of making you believe that everything had truly happened, something I've only seen before in Maggie Smith and Diana Rigg."

Comparison with Maggie and Diana may surprise those who've witnessed Shaw's more recent work. They are more likely to rate her as the worthy heir to Vanessa Redgrave and to expect performances of intuitive brilliance, tears, snot and total self-immolation.She is in now most closely associated with roles (in Electra, Hedda, Machinal) demanding open-heart surgery night after night. But while she turns up her superbly aquiline nose "frighteningly often" at parts "that don't resonate'' (it is rumoured she refused Lynda La Plante's The Governor ), the truth is that she can play anything.

Indeed, she is about to prove herself more astonishingly versatile than anyone but Deborah Warner imagined. As King Richard in Warner's production of Richard II, she will break new ground. Not that there's anything new about cross-dressing in the theatre. When Adrian Lester recently played Rosalind in As You Like It, he was merely doing what boys did 400 years ago; and Hamlet has been played by Sarah Bernhardt and, more recently, Frances de la Tour. Wood, who cast Felicity Kendal in a boy's part in Stoppard's On the Razzle, says: "Men find it wild and women also find it attractive. It can of course be simply effete. When it works, it heightens the mystery of theatrical sexuality."

"Gender aside, Shaw has all the right qualities, the turning on a sixpence between tragedy and lightness," says Warner, who insists that the casting is pragmatic rather than political. "If you wanted to be political you'd chose a play like Genet's The Maids. In this play sexuality isn't really a problem so we're not going to lose any part of it. Quite simply there's no one else I would want for Richard II. And Fiona must be given these challenges - otherwise it's a waste."

Certainly at this stage in Shaw's career, it is the only Shakespearian role that excites. "I've played Rosalind, the Shrew, Portia, Beatrice and the others - Viola - are included in them; I don't think I'd be going anywhere new by heading in that direction," says Shaw. "Macbeth might already be done in my Electra; Cleopatra I'd love to do in time. I've no wish to do Hamlet. Hamlet is a play about a man and his mother.

"The latent prompting of Richard II is that my femaleness is very near to the heart of the play, and the world of the play is very near the heart of the theatre. It's funny, I'm asked this question about why a woman, but you might just as well ask why somebody who isn't English is playing him, or what makes us in the 20th century think we can play people from other centuries. The theatre's role is to show things by reflection not by direct representation. I believe that you could do a production where you get down to the naked woman but people will still see a king because that's what they believe they see. That's a terribly good jolt for our accepting minds." Shaw says thiswith the questioning cleverness of a mind sharpened by a philosophy degree ("a perfect way for helping someone as scatty as me focus on text"). And she continues: "What is the nature of the theatre? To create totalities that people can look at, accept, reject, be changed by. Why paint the painting that's already been painted? Paint the one you don't know. There's no harm done. Next year you can put somebody nice in it and have the play back again if you need to."

The cast persist in calling the King "she", a semantic blip that delights her, but then there's nothing remotely male about her. Her hair-cut - a closely-cropped skull-cap - is vaguely androgynous, but her frequent warm gusts of giggles have a hearty girlishness and, as if to prove she's every inch a woman, she pulls up her rather ragged T-shirt to show me how much flab she is determined to run off. But beneath the jolly-hockey- sticks bounciness is a drive and intensity common among professional women who live alone, rejecting the comforts and confinements of domesticity in order to dedicate themselves to their work.

She is talking about working hard, something she learnt to admire from the example of her father, a Cork eye-surgeon who was on call on Sundays. "In Ireland to work Sundays seemed always to be the most lofty duty anyone could perform. I often work Sundays and late into the night. It matters to me that my father perceives I'm working hard because the theatre is the last great circus and there's a sense of running away to join it. This morning I was running in dappled sunlight down the canal at Primrose Hill, and there was nobody there because everyone was at work. Of course it's a most privileged way of life - and it's sometimes good to notice that because the activity itself is not pleasant, it's terribly exposing and you meet yourself very quickly... we're heading straight now for Pseuds' Corner. It's quite hard to know there's something you're not getting and you must try and get it, and your roots to it are not private humiliations because there are people observing the endless culs-de-sac that you pursue totally and rigorously and then daftly have to relinquish. There is an astonishing alchemy between one's imagination and the text. Sometimes the play takes you up an alleyway you'd never have thought. It's mind- travel, isn't it?" She says this with charming self-parody.

The culture of performing was an integral part of Shaw's middle-class Cork childhood. "I don't mean turf fires and folk dancing - I mean people entertaining others. Whenever I go home my mother charges into the drawing room and sits at the piano and plays something gorgeous." Naturally, Shaw learnt the piano, the cello and to recite poetry and did it all extraordinarily well. "It's cliched stuff really but when you're good at something it encourages you no end. I was also fairly good at tennis..."

Shaw couldn't possibly be English. Her facility with language and bold lack of restraint would instantly establish her Irishness if her delicious accent didn't give her away. "Maybe as an exile I am less protective here than I would be in Ireland," she says. "But I can plug into this thing or out of it. I'm sad not to have a bigger relationship with Ireland. If I had time for a wound I would get wounded about it. But I love London and there are people and places where this kind of experimentation that I am doing can be served."

She has given America a whirl doing various "bummy movies" which haven't so much as scratched the surface of her talent (much to her disappointment). Invariably playing a villain (casting directors rarely see beyond her nose) and usually poured into a vampy spray-on dress, she has had a good time for a week or two and then "got incredibly homesick for conversation". While shooting Super Mario Bros she invited the cast to Shakespeare readings at her beach-house in an attempt to keep herself imaginatively alive. "They do so much hanging-out - take-aways and movies - I tried it and I go mad. I think it's a language thing. I love hanging out here if it's with someone thoughtful."

Which brings us back to Deborah Warner, with whom she hangs out to do her electrically best work. The relationship works, she explains, because they are at once formal and intimate with one another and merciless with themselves. "Deborah is just delighting herself. She has a completely 17th-century relationship to the world. It's theatre but it could have been quilting or statuary," she laughs. "She's very English, intensely calm - the Empire was built by such people. And me? I don't know. I'm just so relieved when I hit on something that's correct. That's what it's about."

n 'Richard II' is now previewing at the Cottesloe, National Theatre, London SE1 (Booking: 0171-928 2252)