It is hard to understand now the shock The Royal Hunt of the Sun caused in 1964. Here was a stylised historic spectacle dealing with conquests, massacres, Inca sun-gods and looted gold in a theatre dominated by Beckettesque minimalism and box-set naturalism. Indeed, Peter Shaffer himself, through West End hits such as Five Finger Exercise and The Private Ear and the Public Eye, had become the favoured dramatist of Binkie Beaumont, who ran the Shaftesbury Avenue firm of HM Tennent like a quality grocers. So it was perfectly natural that Shaffer would send his new play to Beaumont.
"I was staying at Binkie's country cottage for a weekend," says Shaffer, "having heard nothing from him about the play. I went to get myself a drink and was just about to go into the main room when I overheard Binkie and his partner, John Perry, discussing it. I heard John say to Binkie, 'And then the Spanish soldiers go up the Andes.' And Binkie said 'They do what?' John replied 'They climb the Andes, dear.' 'And what do they do then?' asked Binkie. 'They climb down the other side,' said John. To which Binkie simply said 'Fancy!' At that moment I thought perhaps I hadn't sent the play to the right management."
But the stage direction that horrified Beaumont excited the imagination of John Dexter, who ultimately directed the play for the National at Chichester. Dexter was a combative genius who, as Shaffer acutely remembers, always had a note of challenge in his voice.
"Dexter rang me, told me he'd read my play and liked it. He said he was busy directing Larry [Olivier] in Othello but would be with me the next Saturday to read act one of the Royal Hunt aloud and the following week to read act two. I said I supposed that would be helpful. Dexter said, 'If I do it, it will be.' And so it was, with John inviting me to give notes after each scene. But just before we got to the moment where the conquistadors climb the Andes, I said I ought to say a word about the next page. Dexter instantly said, 'If you take that line out, I'm not directing.' Dexter was sending me a message that he was the right man for the job and actively embraced the break with naturalism."
The Royal Hunt of the Sun deals with the violent Spanish conquest of Peru and a clash of civilisations. In light of the American invasion of Iraq, has it become a different play today? Less exotic, more political?
"The political resonance was always there," says Shaffer, a little deaf as he nears 80 but still boyishly high on the adrenaline kick of rehearsal. "The Spanish said they were going to save the Incas from savagery and idolatry and make their life better because they'd have Christ: today we offer democracy as a panacea. And, while the conquistadors were blatant in their admission of greed, today the need for oil has replaced the hunger for gold. I'd be willing to bet that any incursion throughout history in which the invading country has proclaimed it is bringing benefits to the conquered is based on a lie.
"Far from learning anything from Inca society - which, although very static, had a pension system unknown in Europe at the time - the Spaniards also committed unbelievable acts of destruction. People who had landowning status before were sent down the mines, and all the best perks went to the new rulers. Aesthetically, all the great achievements of Inca gold and silverwork virtually disappeared. In much the same way, Baghdad's museum treasures have been vandalised under American occupation."
It will be fascinating to see to what extent Trevor Nunn's new Olivier production combines emblematic spectacle with political inference. Nunn starts with the advantage of never having seen Dexter's original and with the presence of two fine actors, Alun Armstrong and Paterson Joseph, as the conquering Pizarro and the Inca sun-god, Atahualpa.
Not only did The Royal Hunt liberate Shaffer from naturalism, it also set the pattern for later plays such as Equus, Amadeus and Yonadab, in which an envious, rationalist outsider yearns for the instinctual ecstasy unjustly bestowed on another. But how much does this recurring conflict between Apollo and Dionysos stem from Shaffer's own life?
"I was brought up in an Orthodox Jewish household," says Shaffer. "I don't think I ever had a single discussion with my parents about faith. It was just something gently imposed. It was strangely cosy and reassuring but I can't say I lost my faith because I never really had it in the first place. It was more a matter of going through the observances. As for the conflict between the Apollonian and the Dionysiac, I admit that it's an obsessive thing, but I suppose part of me is always looking for a pre-selected meeting of opposites even if they're not always antithetical. You can have a conflict between two different kinds of right."
It is not difficult to translate Shaffer's childhood religious memories into a passion for theatrical ritual. But the capacity for envy that drives the psychiatrist in Equus, or Salieri in Amadeus, is more obviously personal, and something of which Shaffer speaks with silvery candour.
"It's an enactment", he says, "of my own internal tension. A part of me is always envious of people who live in the present and are sustained by a sense of spontaneity. Even dogs have that capacity: they're always wanting to participate in something and I don't often have that element in me. But I envy people who can dissolve themselves in the moment and surrender to their Dionysiac instinct. I also live between the twin poles of admiring both ardent rationalists and sincere believers, though one has to be careful about what one is admiring: I've known fanatics who lead incredibly narrow, warped lives. But I do envy people of a quiet and lambent faith. I can't remember who wrote that. Perhaps I did?"
Intriguingly, that sense of emotional detachment that haunts Shaffer finds its echo in many other modern dramatists: Simon Gray, Christopher Hampton and Tom Stoppard most prominently. It also clearly reverberates with audiences around the world. But, if Shaffer expresses the dilemma of the self-critical observer envious of the scientific and religious certainty of others, he has his own compensations.
"Art and literature," he says enthusiastically, "are my surrogate religions. I find in Mozart that ecstasy I don't find in codified faith. I also find in reading - and even sometimes seeing - Shakespeare that same pleasure in perfection I discover in Mozart. When I read the last act of Antony and Cleopatra and that speech beginning 'The crown of the earth doth melt' I feel I'm encountering one of the great achievements of mankind. It's a beacon somehow, a reminder that there is a perfection of art - whereas I don't think there is a perfection of religion. I wish I could say I found this in the theatre. Not so long ago I saw Troilus and Cressida, and when we got to: 'The time scants us with a single famished kiss, Distasted with the salt of broken tears', there was no sense of the actor being aware of the lines he was privileged to say."
If Shaffer finds in Shakespeare that transcendence he can't find in religion, he is not alone. But Shakespeare also possesses another of Shaffer's most prized qualities: narrative excitement. In fact, Shaffer tells a great story about a schoolmaster he encountered when he was nine. It was a gloomy, wet Friday afternoon and the teacher offered to tell the class a ghost story. It began on windswept battlements at midnight. A ghost appeared in chainmail and told the hero that he didn't die a natural death but was murdered by his brother. On went the story to the point where the murder was about to be re-enacted in front of the brother, who was now king. At which point the teacher broke off and said, "Good heavens, it's three o'clock. We'll have to finish this next Friday."
"I suggest," says Shaffer, "this was the best piece of education I ever had in my life. I had no idea this was a play called Hamlet. The point was that neither I - nor the rest of the class - could wait till the next Friday. I became respectful of narrative and great stories through Shakespeare. I hate it when Brecht says that we should not be interested in the next scene because it distracts us from the current one. I find that priggish and tedious. I want to be enthralled and Shakespeare teaches one an immense amount about how to organise a story; or sometimes how not to. I've always felt Much Ado About Nothing badly needs a rewrite."
In that sense, Peter Shaffer is a traditional writer: a story-teller who learned the craft of narrative by studying Shakespeare but also by co-writing three detective novels with his twin brother, Anthony. One of them, Withered Murder, even has a Shakespearean ring to it. But in another way, Shaffer is the very antithesis of the safe, commercial dramatist. The Royal Hunt of the Sun, Equus and Amadeus brought ritual, magic, music and choreographed movement back into a theatre that was in danger of succumbing to monochrome naturalism. Shaffer has created his own particular, paradoxical niche: that of the popular experimenter and the doubting rationalist yearning for a god in whom he can't finally believe.
· The Royal Hunt of the Sun opens at the National Theatre, London SE1, on April 12. Box office: 020-7452 3000