I run this theological anecdote past Fiona Shaw. Once she has stopped laughing, she explains with due modesty that, far from being the author of her own refutation, she was merely familiar with St Anselm's. All the same. You can blame her degree in philosophy from the University of Cork, but is this stuff actually useful when playing, say, Dennis Hopper's wife in Super Mario Bros? ("Oh God," she groans, "why did I do Super Mario Bros? You never realise the extent to which it's going to come back and haunt you. You think it's going grape-picking.")
The issue of Shaw's erudition seems relevant now that, after 10 years spent carefully cherry-picking her way through the tragedies of Hedda, Electra, Richard II and co, she is about to open in an adaptation of The Prime of Miss Jean Brodie. Muriel Spark's iconic creation, the Mary Poppins of the blackboard, who plies a receptive group of Edinburgh schoolgirls with extra-curricular knowledge, marks a departure in Shaw's stage career in almost every respect. The Prime of Miss Jean Brodie is by no means a classic play; Shaw frankly calls it "a very prosaic text". Nor is it a particularly old one: it's set in the Thirties, but the original novel was written in 1961, and adapted for stage and screen, by Jay Presson Allen, later in the 1960s. Yet Miss Jean Brodie may prove to be Shaw's most fitting role.
For the first time Shaw will be playing someone as top-heavily educated, as over-qualified for her work, as herself. She calls Miss Brodie "a force of nature" and "a priestess of the imagination". It's all too easy to pin these descriptions on Shaw herself, and not just because of the hypnotising density of her performances. She's like that here, off-duty, off-stage, apparently off-guard. You could never tell from the sheer austerity of her work in the 1990s (let's draw a veil over Three Men and a Little Lady, the only other time she has played a teacher) how jolly she is, nor how pretty. She has the scrubbed face of someone who leads half the cast on a daily lunchtime jog over Blackfriars Bridge, along the north Embankment and back over Waterloo Bridge. "Or Hungerford Bridge, if we're feeling really virtuous."
Other common ground between Miss Shaw and Miss Brodie is probably just as coincidental, but no less compelling. For a start both are unattached and live alone, leaving all the more time to be almost messianically committed to their vocations. They both come from outside the capital cities in which they have made their home. One fancies that the young Scottish actresses in the cast look up to Shaw much as the "creme de la creme" they are playing put their faith in Miss Brodie. Shaw has even joined Miss Brodie's profession: she was recently made Professor of Drama at her Alma Mater in Cork.
The other thing is that they are both inextricably linked with Maggie Smith. Shaw's aquiline hauteur and gift for comic timing has drawn frequent comparisons with Smith, who played Miss Brodie in the 1969 film. "I'm an enormous fan of Maggie Smith," she says, "but I think comparisons are odious. They tended to be made early on - when I was particularly interested in comedy. I've done no comedy for 10 years and the comparisons have calmed down in the interim." In fact that's not quite true. Two years ago, at the National, Shaw cut an elegantly disdainful figure as Millament in The Way of the World, another role over which Smith casts a long shadow. Whether to avoid replication or not, Shaw has chosen not to revive the aerated Morningside accent of Smith's Brodie.
In preparing for the role, Shaw visited Rome with director Phyllida Lloyd "to walk the turf of Miss Brodie's holidays". It's in Rome that Miss Brodie, Presbyterian by nurture but italophile by nature, develops a taste for the theatricality of fascism which will eventually cause her downfall. To get a feel for the period, Lloyd took pictures of Shaw wearing Thirties dresses at the Colosseum. They then caught the train up to Arezzo to have lunch with Miss Brodie's creator in her Tuscan retreat.
"It was everything that you'd ever wanted when you get to the Aladdin's cave of a writer," she says. "It was very exciting being with this person whose imagination is so violent and whose manner is so graceful. I did find it very helpful. Dame Muriel explained that Miss Brodie was a Catholic in all but name. But it's not a theological position really; it's more of an aesthetic delight. She talked about Miss Kay [Spark's own teacher, on whom she based the character] having that very good taste in Cimabue and Giotto, when you get that blend of civilisation meeting earth colours that later became rather high-blown. She also said that she knew when she wrote the book that she was hitting a target because everybody had a Brodie figure."
At day school in County Cork in the 1960s, Fiona Shaw had three Brodie figures. "They made the subjects that were hithertofore boring suddenly delightful, and the line between pleasure and learning was blurred." She recalls meeting Shakespeare with "a Miss Emily Fitzgibbon", doing elocution with an anarchic teacher called Abbie Scott. But it was Miss O'Rourke who made the deepest impression. "I remember thinking, I will never have read as much as Miss O'Rourke has by 28, and now I'm 39 and I don't think I have read as much. You're being opened. It's this walled garden of massive fertility and you can't believe that you could go in there if you could just work hard. She got us to connect literature with life, and she did this other thing that Miss Brodie does. She did not talk down to us, and maybe pitched the classes slightly above our abilities, so we were always grasping up."
That instinct to reach beyond her grasp seems to have stayed with Shaw, and it probably explains why at least on stage, Brecht and Eliot are about as modern as she gets. You get the impression she'd be a caged lion in an Ayckbourn play or even a Stoppard. When I put it to her that she could be seen to be avoiding the new wave of plays emanating from both Britain and Ireland, there is a rare moment, of silent, nervous contemplation. "This is a minefield, isn't it? I'm an enormous fan of Sarah Kane, and of the Royal Court in general. But goodness me. It's pure coincidence." Then she takes a deep breath and adds, "The period in which big plays and big ideas were fused in big characters is not the period we've been in. So if you are a heavyweight boxer, you need something to punch."
The most modern original play she has appeared in was Machinal, written in 1938, about a stenographer who marries then murders her rich boss. She compares it to "doing a starvation diet. It's quite interesting doing these plays where the writing can only barely hang on to the size of the situation." In other words, there's an implication - which Shaw would be loath to spell out - that she is simply too Himalayan an actress for the footling issues bandied about in modern theatre.
It may also be a foreign area for her because a lot of modern plays handle sexuality, and sexual passion is one theme Shaw has rarely investigated. It's telling that when she did two productions of As You Like It back to back in the 1980s, she was "much happier" playing the mordant ironist Celia than the gender crossword-puzzle that is Rosalind. This suggests that when it comes, her Cleopatra will be a tough challenge. "I'm very off Shakespeare at the moment," she says. "One day I would love to play her. I suspect she includes many elements of really everything that one has played. And I've always had it in my mind that I'll give up Shakespeare after that."
So what about Irish theatre? Shaw is the most famous Irish actress outside Ireland, though less so within it, despite her roles in My Left Foot and The Butcher Boy. Doesn't she hanker to play a character who shares her nationality onstage? "I would love somebody to write a big Irish play. I would be in it. I'm available. But there is no play that has dealt with the world I'm from. There is an enormous middle-class Irish population who are quite well-educated and have led perfectly decent lives and never get reflected in plays. That's the play I need, and I wish somebody would write it. But maybe it's not the place for plays. It has ironed out the difficulties - not psychologically, but it isn't a troublesome area." (Or, indeed, Troublesome.)
Shaw belongs to that Irish generation which joined the brain drain out of the country. She describes Miss Brodie's Europeanism as "a need to be fed by a horizon wider than Scottish Presbyterianism". I wonder if she had a comparable need to be fed by a horizon wider than Irish Catholicism. "I don't think I had a philosophy about it. I suspect the rocket that shot me out of Ireland was a youthful need for life. I just wanted to be away. There was a sort of tide. Remarkably, it's changed and it's great that it has. They are having an enviable time. Oh to be young and Irish now. I very much want to re-root myself a bit more back there. Buy a house and live there sometimes." She concedes that after nearly 20 years away, she does tend to glamorise Ireland. "It represents memory and ease, and of course it does genuinely hold emptiness: it is quieter."
A quarter English, she also has an interesting theory that in the end, the English and Irish are singing from the same hymn sheet. Both nations suffer from the disease of irony. "And it is a disease, because we refuse to deal with anything. Of course, the upside is that it makes us charming, and both the English and the Irish are very charming, because irony has an ability to not take anything too seriously. We find fault with anyone who hasn't got irony, and they always look at us and think, what are those people doing flicking language when they could be saying something profound?"
Two years ago, Shaw decided to swaddle her innate sense of irony as completely as anyone can when, in search of a spell of inner peace, she spent two weeks in the Tyburn Convent on the Bayswater Road in west London. "It was just looking over the garden fence, thinking, that's what I'm missing. I have to say I have revised my feeling about it a hundred times since. For a while I was despairing at the abject banality of the nuns' lives. Then I began to think that beneath that there was a secret I was missing, which is that they fundamentally are on to something of such
eternity. You suddenly feel like a flapper. In their faces there was a sort of clarity that we in our tortured lives will never know. But it was awful, actually. They're not interested in the world." Worst of all for Miss Brodie, and for Miss Shaw, the nuns, in their silent, lifelong rehearsal for the vacuum of death, don't even read. Not even St Anselm.
'The Prime of Miss Jean Brodie': Lyttelton, SE1 (0171 452 3000), previews from Friday, opens 25 June, to 3 October.