Ian McEwan looks a bit shellshocked as he steps out of the first rehearsal of For You, the opera he has just written with the composer Michael Berkeley. “No words of mine have been sung for 20 years,” he explains. “And, on the whole, it doesn’t suit novelists to be collaborators. We are so used to playing God by ourselves.”

But surely McEwan has collaborated quite a bit in films – not least of his own novels, including Atonement? “Yes, but movies are odd collaborations because writers are so disposable. The last movie I wrote I got sacked from, and it still got made without me. So I thought: ‘That’s enough of that!’ Opera is a much classier collaboration.”

He has known Berkeley for 30 years. Berkeley’s wife is McEwan’s literary agent, and the two men collaborated 26 years ago on a pacifist oratorio, Or Shall We Die?, which is being revived in Wales just a few days before Music Theatre Wales gives the new opera its premiere later this month. McEwan seems to trust Berkeley absolutely. “You write such good tunes,” he tells the composer, “that one day we’re going to force you to write a musical.”

Even so, like every opera librettist in history, he is already fretting that his words won’t be heard when For You finally gets on stage. “My one stipulation was that there should be surtitles, even though the opera is in English,” he says. “It seemed crucial to me that the audience knows exactly what’s going on as it happens, not from some vague memory of a synopsis that perhaps they read while downing their drinks and hurrying into the auditorium.”

“I have no argument with that,” Berkeley says. “With a new opera there’s so much to take in. And the truth is that once any soprano sings anything above high F or G, it’s very hard to hear the words, no matter who the soprano or the composer is. So what does a composer do? Not write anything that high? That would be to castrate oneself, musically speaking.”

Having read McEwan’s libretto, I agree that it would be a shame if the audience missed a single word. It’s a corker: ironic, blisteringly funny, fast-paced, pithy and with a surprise comeuppance for the ghastly main character that makes the end very dark indeed. Typical McEwan, in other words. And what gives it a special piquancy is that the chief protagonist is an ageing composer/conductor who is obsessed with revitalising his failing powers by humiliating, then seducing, young female musicians in his orchestra, even as his wife is having a serious operation.

An egotistical serial-philanderer? Not like any real conductor, then. “Well, I can certainly think of one or two candidates, as I’m sure you can,” Berkeley giggles. “But this is essentially fiction.”

“I certainly had nobody in mind,” McEwan confirms. “I just thought that a man with such power is bound to abuse it in some respect.”

And should we forgive such abuse, if the man happens to be a creative genius? “No,” McEwan says. “I think that the concept of ‘artistic temperament’, and the notion that you give some special licence to creative types to behave badly, is complete bullshit.”

So McEwan wouldn’t approve of Wagner? “Certainly not. I much prefer Bach. Good family man!”

Whatever else For You is, it’s unlikely to be called family entertainment. The startling ending, involving a Polish housekeeper with an unrequited passion for the conductor, is as black as anything in Hitchcock. The language is fruity (McEwan expresses astonishment that the opera world is still squeamish about using the F-word). And, as with most things to come from his pen, sex is a big element. Indeed, one sung exchange hinges (as it were) on the composer’s inability to conjure an erection with his new mistress. “The point is that a man may be able to fake an orgasm, but not an erection,” McEwan explains.

Berkeley looks worried. “Actually, Ian,” he says, “on stage I think we may have to!”

Of course, writing an opera about a composer/conductor does open up all sorts of “play within a play” possibilities. At the start of the piece, for instance, he will descend into the orchestra pit where the band is tuning up (and where the “horn player” he will seduce will be planted in the brass). “I also use that ‘tuning up’ – which is all composed – later as a metaphor for confusion and chaos,” Berkeley says.

And near the end there’s another clever blurring of musical fact and fiction when Charles, the composer, conducts his new premiere – a piece called Demonic Aubade, which sums up his ruthless art-at-any-cost attitude to life. Writing such a piece must have posed quite a challenge to Berkeley. “Yes,” he agrees. “The trouble is that I actually found Ian’s words here quite uplifting, whereas I knew that he wanted them to sound overblown and distasteful.”

“I felt that this was Charles trying to defend his behaviour and grossly overstating his case,” McEwan adds. “But I accept that we couldn’t have had ten minutes of terrible music at the end of the opera, just to make that point.”

In the opera the sixtysomething Charles feels disconnected from the music he wrote in his youth. Can McEwan and Berkeley, who both turn 60 this summer, relate to that? “All creative artists feel a growing distance between their 60-year-old and 20-year-old selves,” McEwan observes. “It’s tempting to disown the stuff you wrote in your twenties, because it’s no longer what interests you. The paradox is that this stuff exists in the present tense for everyone else. So you have to stand by it. And of course a really big success early on can skewer you.”

These reflections are particularly pertinent to Berkeley and McEwan at present, with the revival of Or Shall We Die?, which they wrote back in 1982. “Ian says that there are some things in it that he wouldn’t write now, and my music has certainly changed utterly since then,” Berkeley says. “But in the end we decided not to change anything. It’s a record of how we were and what we felt at an earlier phase of our lives.”

But will the piece have the same shock impact now? After all, this cri de coeur against the stockpiling of nuclear weapons by the super-powers was written when the world was very different.

“The situation with nuclear weapons is as dangerous now as it ever was,” McEwan claims. “It was only a few years ago that India and Pakistan were squaring up for a war. Saudi Arabia might acquire them fairly soon. Iran obviously. North Korea has. Israel has. Do you think we will get through the 21st century without a nuclear weapon being used? I am pessimistic.”

So McEwan stands by what he wrote 26 years ago? “Yes. Except that in 1982 I had the rather romantic notion – which many writers hold at some point or another – that the problem with the world is actually men, and that everything would be all right if women ran it. I no longer hold that view.”

For You opens at the Theatr Brycheiniog, Brecon, on May 31 and then tours (musictheatrewales.org.uk).
Or Shall We Die? is performed at St David’s Hall, Cardiff, on May 22 2008 (0800 0521812)

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