Don't expect a new stage version of 'Amadeus' to be like the film. Peter Shaffer tells Arminta Wallace why he has rewritten it so much

It was the laugh that did it. Remember that awful, asinine snigger? It helped turn Amadeus, Milos Forman's 1984 film, in to a box-office sensation, and it turned the world's image of Mozart upside down. Instead of an eternal child, seated at the virginals playing cute little duets with his sister, Amadeus gave us an infernal teenager, all spots and snorts and dodgy double entendres. It also gave us the character of Antonio Salieri, the brooding, embittered court composer who, according to the film, was Mozart's great rival.

The extraordinary success of Amadeus made Salieri a household name for the first time since the late 18th century. Even now, 20 years later, people who think Don Giovanni might be the name of a brand of olive oil can tell you that Salieri was the guy who poisoned Mozart. Salieri, of course, did no such thing - and, contrary to popular perception, Forman's film never said he did. But the film is just the best-known chapter in the ongoing saga that is Amadeus.

Peter Shaffer's original stage play was first produced by the National Theatre in London in December 1979, with Paul Scofield as Salieri and Simon Callow as Mozart. It was an immediate success - although not with everybody. In his introduction to the most recent printed version of the text, the director Peter Hall remembers one particularly disgruntled theatre-goer.

"Margaret Thatcher," he writes, "was not known for her enthusiasm for the arts. She visited the National Theatre only once during my 15 years as its director. Unfortunately, it was to see Amadeus. She was not pleased. In her best headmistress style, she gave me a severe wigging for putting on a play that depicted Mozart as a scatological imp with a love of four-letter words. It was inconceivable, she said, that a man who wrote such exquisite and elegant music could be so foul-mouthed. I said that Mozart's letters proved he was just that: he had an extraordinarily infantile sense of humour. In a sense, he protected himself from maturity by indulging his childishness. 'I don't think you heard what I said,' replied the prime minister. 'He couldn't have been like that.' "

If Amadeus made an indelible mark on popular culture, its impact on the world of classical music has hardly been less impressive. In his book 1791: Mozart's Last Year, the American Mozart specialist H. C. Robbins Landon describes Amadeus as legendary. And Maynard Solomon's enormous, and enormously influential, 1995 biography Mozart: A Life refers in passing to "the author of Amadeus" - a tacit acceptance that when it comes to Mozart's life and work, Amadeus is as much a part of the story as anything else.

But which Amadeus? This evening, when Ouroboros theatre company - formerly Theatreworks - takes to the stage of the Samuel Beckett Theatre, at Trinity College in Dublin, with the first production to be seen in Dublin for two decades, Denis Conway (as Salieri) and Patrick Moy (as Mozart) will be performing in a play that differs substantially not only from Forman's film but also from the original stage version. This is because Shaffer has reworked Amadeus a whopping six times, transforming it, in Hall's words, from "admittedly thrilling melodrama" to "a profoundly humanist play about forgiveness and atonement".

It was the climactic - and, of course, entirely fictional - scene of confrontation between Mozart and Salieri that apparently caused all the trouble. In an entertaining essay in the latest printed version of the play, Shaffer revisits his six rewrites - of which the film is the third and, according to the author, "utterly improbable" version - and charts the shifts of dramatic emphasis they brought.

He also recalls some of his more outré inventions along the way. These include, in the first version, the addition to Salieri's household of "a cadaverous and fanatically religious" valet by the name of Greybig; in the second, a moment when Salieri chews a piece of Mozart's Requiem and spits it out at its composer; and, in the third, played on Broadway by Ian McKellen and Tim Curry, a confrontation that ended with "Wolfgang scuttling under a long worktable, desperately singing his father's little bedtime Kissing Song to the tune of Twinkle, Twinkle Little Star, with Salieri yelling down through it from above, 'Alone! Leave me alone, ti imploro! Leave me alone at last!' "

It will be interesting to see how many, if any, of these feature in Michael Caven's new production. It is also a moot point whether, given that he has already rewritten it so many times, Shaffer will ever be quite satisfied with Amadeus. Is there, perhaps, something inherently unsatisfying in the dramatic relationship between Mozart and Salieri? Some problem that can never be fixed? On the phone from New York, a relaxed Shaffer says not. He is, he declares, perfectly happy with the sixth and - so far - final version of the play. In this one Salieri "realises that 10 years of unrelenting spite have destroyed his own life" and asks the younger man for forgiveness, which Mozart, already in his death throes and rapidly regressing in to childish avoidance, is unable to grant.

Can Shaffer remember, after all this time, why he was attracted to the subject in the first place? "I've always been a tremendous lover of Mozart," he says. "That must be obvious, I suppose. And I've always been fascinated by that strange story about people being turned away from his funeral by a great storm. When people ask me about the genesis of Amadeus I often say it was a weather report."

While researching the story of Mozart and Salieri he came across contemporary reports from Vienna's meteorological office and from the diaries of Count Zinzendorf, a socialite of the day.

"His passion was keeping meteorological records - God knows why," says Shaffer. "But anyway, according to both sources, the weather on the day of Mozart's funeral - December 6th, 1791 - was exceptionally mild. There were no storms. And I began to wonder why these five people who attended the funeral had come back and reported that they couldn't get in to the cemetery because they were driven back by a storm.

"I began to wonder, in an Agatha Christie sort of way, what the real reason was. Was it the secret police? Perhaps, if it was believed at court that Mozart had been poisoned. In the last days of his life he had been saying that he had. Perhaps someone at court had said: 'Listen, if this is true we'd better make sure nobody knows where he's actually buried, just in case.' Then I discovered who the five people were."

Four were musician friends of Mozart's, "and the fifth mourner was Salieri".

The enigma was irresistible.

But Amadeus was never a whodunnit. On the contrary, says the playwright, it sends up the idea of a murder mystery at the very beginning, when Salieri says: "The death of Mozart - or did I do it?" Which, Shaffer notes, always gets a cosy laugh. The current thinking among musicologists, backed up by the scant forensic evidence available, is that Mozart died of a streptococcal infection. Salieri may have hindered the younger composer, frustrated him at every turn - hounded him to an early grave, even - but he did not poison him.

In recent years Salieri's musical star has been on the rise, culminating in the release last year of an extravagantly praised album of his opera arias by the Italian superstar Cecilia Bartoli. How much of this belated interest is due to the influence of Amadeus?

"Oh, everything," says Shaffer. "I mean, a lot."

There is some truth in this, but it's also true that Mozart overload in the CD catalogues, coupled with an apparently inexhaustible appetite among the CD-buying public for polite, well-bred 18th-century music, has driven young artists to search outside the mainstream for works to record.

Ultimately, does this enthusiasm for Salieri's music put the kibosh on Shaffer's portrayal of him, in Amadeus, as the patron saint of mediocrity?

"Not at all," says Shaffer. "I think it is average, his music. I mean, there are some showy arias, but it's average music of its day. It's agreeable music, certainly, but there are none of those heart- stopping moments which abound in Mozart.

"Of course, Mozart wrote a good deal of formulaic music as well. But the music of his last 10 years is quite extraordinary. It's one masterpiece after another after another. And you can't give an emotion to it. You can't categorise it. Ultimately, there's only one word for it. It's Mozart."

Looks as if Amadeus gets the last laugh after all - unless, of course, there's another rewrite.

Amadeus opens at the Samuel Beckett Theatre, Trinity College, tonight