Who can hit the right note for a winner?

'The Full Monty' kicked off on Broadway last week to much acclaim. But successes are hard to predict

On Thursday night, after a sell-out run in San Diego, the stage version of The Full Monty opened on Broadway. Back in London, producers and cast of the epic new musical Napoleon are still reeling from last week's critical savaging. From workers taking their clothes off to the coronation of the French Emperor, musicals keep pouring out in ever more unlikely shapes and sizes.

On Thursday night, after a sell-out run in San Diego, the stage version of The Full Monty opened on Broadway. Back in London, producers and cast of the epic new musical Napoleon are still reeling from last week's critical savaging. From workers taking their clothes off to the coronation of the French Emperor, musicals keep pouring out in ever more unlikely shapes and sizes.

Just take a glance at the current crop. Three East Coast divorcees get seduced by a devilish figure. A group of Belfast teenagers play football against a background of sectarian violence. A small Corsican rises from obscurity to become the most powerful man in the world. How could you possibly tell which of them would succeed and which of them would flop?

The Witches of Eastwick has been acclaimed as "devilishly funny", The Beautiful Game as Andrew Lloyd Webber's "finest piece of musical theatre ever" and Napoleon described as "tosh", "cartoon-history", "a veritable mish-mash of triteness, clichés and dodgy rhymes", and "preposterous even by the standards of big, sentimental musicals".

There may be students of history who think Napoleon's most decisive moment was routing the Austrians and Russians at the battle of Austerlitz, but for the writers of the musical Napoleon the world was changed "for ever" when their lead character first asked Josephine to dance.

It's a reminder that the success of a musical doesn't lie in the subject itself but in how you choose to present it. Puccini tried to set Oliver Twist to music, but it was Lionel Bart who gave us Oliver!

The Dome should really be playing host to an exhibition of the great bad musicals. It would want to include extracts from Moby Dick, which had a school group trying to stage a production of Melville's novel in a swimming pool; Bernadette, the story of a peasant girl who saw visions at Lourdes in 1844; and The Fields of Ambrosia about a travelling executioner in the Deep South who falls in love with his first female client. If there was space - and there would be at the Dome - you would want to see bits from The Hunting of the Snark by Mike Batt, composer of "Remember You're A Wom-ble", the Norwegian pop opera Which Witch? and the RSC's production of Stephen King's novel Carrie, which inspired a book about theatrical disasters called Not Since Carrie.

But the successes sound just as preposterous as the flops. When Trevor Nunn was rehearsing Cats he found that colleagues in the theatre could barely disguise their amusement at the seeming badness of the idea of a musical based on Old Possum's Book of Practical Cats. Evita didn't sound very appealing either. One person who wasn't keen was Andrew Lloyd Webber. He had just done Jesus Christ Superstar and didn't want to do another show about an unknown who rises to fame aged 33, and then dies.

The book-writer and lyricist Michael Stewart, whose credits include Hello, Dolly!, 42nd Street and Barnum, used to give lectures on transatlantic cruises. One of his first tips was: Work with Someone who is Dead. The great advantage of collaborating with Shakespeare, Dickens or Victor Hugo is that they don't get tricky at creative meetings. They leave you to get on with Kiss Me Kate, Les Miserables or Oliver!.

It is always easier to sell a product that is known. As Cameron Mackintosh says, shows not based on existing source material "hardly ever become classic musicals". That's why there is no shortage of demo tapes doing the rounds of producers' offices retelling the stories of Quasimodo, Cyrano de Bergerac, Tess of the D'Urbervilles and Adolf Hitler.

Mackintosh gave John Dempsey and Dana P Rowe, the lyricist and composer of The Witches of Eastwick, a list of Warner Brothers film titles that might be available to be "musicalised". Dempsey and Rowe knew what they were looking for: "a structure broad enough, characters large enough, passions oversized enough to meet the needs of the musical stage".

Shows about shows offer a natural plotline as we move from auditions to rehearsals to first-night nerves and the performance. The Full Monty was crying out to be staged. The musical version, adapted by Terrence McNally, has been relocated from Sheffield to Buffalo, New York State. But in its way, The Full Monty is in the same old tradition as 42nd Street - and likely to be as big a hit.

The most remarkable transformation of a movie into a stage musical has been The Lion King. The director Julie Taymor lifted the stage show way beyond the film by drawing on two sources: the Asian traditions of mask, puppetry and perspective, and the emotional depth of South African chants.

It seems simple then: you need the right subject, done in the right way, by the right people, at the right venue. If only that were enough. It also has to be at the right time. Children of Eden was composed by Stephen Schwartz, who wrote Godspell. Its source material was impeccable. It was based on the book of Genesis. It just happened to open during a very cold January on the same night as the UN deadline for Iraq expired.

Timing is all. Chicago is about two women murderers who chase freedom through fame. It enjoyed modest success when it first opened in the 1970s. But the 1990s revival was a smash. The people who should have been on a percentage of box office takings were the regulars on CNN and Court TV - Barry Scheck, Alan Dershowitz, Judge Ito, the Eappen parents, the Menendez brothers, Louise Woodward and OJ Simpson himself. Chicago told us that "in this town, murder is a form of entertainment" and the 1990s audience knew that was true.

After her success with Singin' in the Rain, Jude Kelly is directing Half A Sixpence, a musical based on HG Wells's Kipps, about a draper's apprentice who inherits a fortune. Further ahead, Jonathan Harvey has written the script for a musical comedy by The Pet Shop Boys called Closer To Heaven, set in London with a gay theme. And there's a Bollywood musical on its way, called Bombay Dream, with lyrics by Don Black and the actress Meera Syal.

But my money would be on a musical version of Billy Elliott. Its three biggest scenes are the dance in front of the father, the speech in front of the panel at the Royal Ballet School and the final entrance in Swan Lake. Each one would work a treat on stage. The only question would be whether anyone could write a melody to match Jamie Bell's grin.