Few actresses have gained a seal of intellectual approval from that brain teaser wizard, Stephen Sondheim, but Diana Rigg is in that class. While they waited for Elizabeth Taylor to show up on the set of A Little Night Music (a film apparently so disastrous that it remains under lock and key) Sondheim and Rigg had ample opportunity to test each other's form at word games. "She's brilliant," he said in London some months later.
Now she intends to amuse and astonish us in print. In September next year Hamish Hamilton plan to publish her first book, No Turned Unstoned. It's an idea she has cherished for a very long time, a collection of all the worst/funniest reviews donated by all the well-known actors and actresses in Britain and quite a few in America.
To encourage faint souls, Miss Rigg reveals her own. It was the American critic, John Simon, writing about Abelard and Heloise in New York, including the famous nude scene that tended to confirm that Miss Rigg, from RSC graduate to Emma Peel of The Avengers, would do anything, go anywhere, without fear or favour. "Diana Rigg," wrote Mr Simon, "is built like a brick mausoleum with insufficient flying buttresses."
"It is funny, isn't it," she says. "Absolutely nothing to do with the part. But that's the case with these sort of quotable quotes." Gielgud's, a propose his Romeo, is "John Gielgud has the most meaningless legs imaginable."
The book, originally planned as a mixture of more-or-less contemporary quotes and cariacatures, has expanded into a flood of research. Miss Rigg has been through archives of theatres all over London, and has already assembled a vast pile of source material without ever stepping inside the British Museum or London Library.
She has just finished playing the archetypal actress bitch in the new Agatha Christie film, Evil Under The Sun, along with Peter Ustinov, Maggie Smith, James Mason and Jane Birkin. On Thursday The Great Muppet Caper, the second Muppet feature film, opens - also starring Diana Rigg in a kind of Margaret Dumont role. "It's a cough and spit," she says. "I don't think you could even call it a role. You've got to call it a bit."
Playing with the Muppets is another aspect of her appetite for experience. She says she will "try, do anything. It's very easy to tread the classical path. Once you've got to a position in, say, the RSC that I suppose I'd got to by the time I was 25, one could have gone on forever with the narrow, pure, utterly dedicated and not to be diminished, classical career. But that is entirely turning your back on what, as a member of a 20th-century profession, two thirds of your colleagues are doing."
The Muppet job also reflected the fact that, 43 last Monday, she has a four-year-old daughter, Rachael, with whom she had become quite hooked. "I quite understand puppetry leaves some people cold. But what they do is a kind of magic, And again it's an aspect of the theatre which has died out for the most part. What they achieve now is extraordinary - Miss Piggy dancing like Ginger Rogers with 12 gentlemen in top hats and tails. It's nice to a part of that, even if the human beings are just feeds."
She really is not a film star, she says. What matters most to her is working with a live audience, and she showed how much she cared for that when she stayed with Tom Stoppard's Night and Day in the West End despite crippling back pain.
Because she's so resilient, she gives the impression of having an infallible nose for success, the clever bachelor girl with her head screwed on. In fact there have been plenty of disasters. When she left RADA in 1957, getting work as an actress was far from automatic. Over the next two years she worked as a typist in a blood bank, answered the switchboard for a wholesale clothiers, and waited in coffee bars before getting into rep. Success at the RSC took about two years, and owed much to Peter Brook's casting her as a proud, touch Cordelia.
Each stage of her career has been involved new lessons to be learnt. RADA was no preparation for the new-style Peter Hall RSC, which was no preparation for Emma Peel. One of her biggest flops was Diana, her own American comedy series when the BBC finally bought it, they put it out at 3:30pm.
The important lessons, though, have been learnt in the theatre, from directors like Brook and Dexter. Becoming a mother and getting her to the present age of 43, (she's always been honest about that) have qualified her for a huge new range of mature and tragic parts - for which first Cordelia and later Phaedra prepared her. She has just done the lead in Ibsen's Little Eyolf for television, and will return to the stage in a couple of months in a Shaw play (as yet unannounced).
In the middle of the run of Phaedra Britannica, an older actress in the National company taught her a most important lesson - though it was too late to incorporate it into the rather unsatisfactory Tony Harrison version of the tragedy. This actress had seen Bernhardt play Phaedra in Paris at the age of 12, and recalled turning to her father in the middle of the play and saying, "But Daddy, why is she enjoying crying so much?"
That, says Miss Rigg, is the difference between the French grande dame's approach to tragic acting and ours today. "They adore the drama. They relish the agony. And one is perhaps too 20th century in one's approach to agony. It oughtn't to be allowed."
Next time she comes to play Phaedra, she won't play it in such a racked fashion. Enjoy your tragedy. The same goes for the great Shakespeare roles she would like to tackle. Cleopatra, for instance, in Shakespeare's description, seems to be Diana to a D. "Age cannot wither her, nor custom stale her infinite variety... she makes hungry where most she satisfies; for vilest things become themselves in her, that the holy priests bless her when she is riggish."