With friends like this...

New film: My Best Friend's Wedding, with Julia Roberts, right, out today. Platonic relationships enjoy the most romance, says Ryan Gilbey, in a film lifted to the rafters by its music

A fair way into My Best Friend's Wedding, there is a fleeting moment of blissful surrealism that knocks you for six. Until that fillip, the film has been a baggy pastel-coloured revenge comedy, jazzed up by splashes of farce. Jools (Julia Roberts) has spent the entire story trying to sabotage the wedding of her dearest chum Michael (Dermot Mulroney), who is poised to marry Kimmy (Cameron Diaz), and just when it seems her misanthropic scheme has succeeded, humanity prevails and she tries to convince Michael he was right to choose Kimmy after all. It's a scene pregnant with poignancy.

This is when the director, PJ Hogan, decides to test the sympathies of his audience and the durability of his material. It would be unfair to divulge the specifics, but through a combination of helium-filled balloons and the music of John Denver, dramatic equilibrium is blown out of the water with the elegance of a boozy fisherman blasting at pike.

When a character has taken this long to comprehend the error of her ways, you don't expect the director to go and undermine her big shot at repentance. Hogan specialised in sudden shifts of emphasis in Muriel's Wedding, though in that film it was usually the laughs that were being silenced by tragedy, rather than vice versa. My Best Friend's Wedding features a few choice scenes in which our expectations are abruptly confounded or gently teased, but generally it offers straight trades for anyone who likes their romantic comedy sweet, not salty. Go along with that helium nonsense and in the next scene you get a bona fide emotional showdown, as Jools bares her soul to Michael.

She's a woman who doesn't relinquish her obsessions easily. Michael has been her closest friend since they ended a brief romance years earlier, but now she's shocked to find herself troubled by the imminent wedding. As Rosemary Clooney and Vera-Ellen sang in White Christmas: "Lord help the sister who comes between me and my man."

Ronald Bass weaves some succulent incidental details into his screenplay - the news that one of Kimmy's friends has shattered her pelvis line-dancing, or the scene where a bridesmaid's attempt to fellate an ice replica of Michelangelo's David ends in tears. So it's comforting that Bass gives little credence to a pact which Jools and Michael made, promising that if neither were hitched by the age of 25, they would wed each other. Instead, Jools's quest, indeed her very existence, is fuelled by single-minded, toxic jealousy. What a woman.

Where do you go once the world has fallen in love with you as Cinderella? Sooner or later those glass slippers are going to give you bunions.

It's only after starring in a movie which the entire world has paid to see, twice, that you can afford to squander the best part of your reputation the way Roberts has since she kicked off the thigh-length leather boots of Pretty Woman. In My Best Friend's Wedding, audiences may feel perplexed at being invited to direct their boos and hisses at the woman in whose resurrection they have come to rejoice. After her first encounter with her rival, Roberts bluntly announces: "She's toast." Goodbye, Pretty Woman. Hello, Terminator.

Inevitably, the film reveals itself to be harbouring a message at odds with its main character's motives, so it's no surprise to find that Jools undergoes a defrosting process. Friendship, rather than passion, is the order of the day. To its credit, the picture concedes that platonic relationships have their own unique romance. The most persuasive advertisement for keeping sex out of the equation comes from the scenes with Jools and her gay pal George, exquisitely played by Rupert Everett. While George fulfils the traditional role of adviser and good egg, he remains Jools's sole confidant and the only character with whom the audience is actively encouraged to identify.

Certain concessions have to be made when a gay man emerges as the most sympathetic character in a Hollywood film. Libido is out. George is only permitted to display sexual behaviour when it's in disguise, so he masquerades as Jools's camp fiance - a gay man pretending to be a straight man pretending to be gay - and grabs at every female buttock to perfect his heterosexual caricature. But because you're in on the joke, you don't feel swindled; Hogan wants us to share George's glee at infiltrating Kimmy's celebration dinner and then reeling her coy aunts and uncles into a rowdy singalong with gospel fervour.

Hogan's use of music is the most endearing feature. He draws mostly on Bacharach/David compositions, but he employs them to striking effect. There's the charming opening credit sequence, where a bride and her maids of honour act out "Wishin' and Hopin'" against a bright pink set. Later, Kimmy gets to perform assassination-by-karaoke on "I Just Don't Know What to Do with Myself". It's George's rendition of "I Say a Little Prayer" that gives the film its biggest lift, as the song spreads around the room until the roof of Barry the Cuda's fish restaurant is trembling, and even the waiters and waitresses are waving along with their lobster-shaped oven gloves.