A Streetcar Named Desire, Barbican Hall, London

"We've had this date with each other from the beginning." Stanley Kowalski to Blanche DuBois, or Tennessee Williams to André Previn? Williams always resisted turning his plays into opera - especially A Streetcar Named Desire. They were quite operatic enough without the singing. Then, in 1964, as a goodwill gesture to the composer Lee Hoiby, who underscored his ill-fated Slapstick Tragedy, he gave Hoiby the option to adapt any of his plays (including Streetcar). Hoiby chose Summer and Smoke, but the door was finally unlocked. Blanche DuBois had always been ready for her close-ups; now she'd get her arias.

And who better than Previn to provide them? There are moments in this, his first opera (which has taken nearly five years to reach London, and will be performed only twice) where Previn seems boldly to go where the composer of the Elia Kazan movie score, Alex North, left off. Streetcar - the opera - is full of effulgent Hollywood moments: great swoony climaxes with overreaching strings and descanting horns are relished - and then some - by Previn's old band, the London Symphony.

It's all very 1940s, very noir; the first sound we hear is the ominous lowing of a tuba and a sleazy, oft-repeated, trumpet-laden chord slurred as though through too much heat and booze. Towards the end of this sultry prelude, a solo trumpet slithers to earth like a shattered fragment of New Orleans jazz. There is no real jazz in Previn's score (and no half-baked pastiches, either) - there's just the allusion to it, the smell of it. Take the brilliant moment at the close of the first act, where Stella (the lustrous Janice Watson) sings a lazily contented, insidiously memorable kind of scat which tells you how much better sex is with Stanley after he's knocked her around. That's pretty unsettling.

Then again, you could argue that the problem with Previn's Streetcar is that it's too sophisticated, too lush, too lyric, steamy rather than seamy, not down and dirty enough - that its European airs poop the New Orleans party. Certainly, the vocal writing is sensuous even where the words are grubby. The librettist Philip Littel has done a fine job of whittling down the meat of this great play into a cogent, well-proportioned dramatic entity. All your favourite lines are just where you last left them. But Previn hasn't always found a way of characterising them vocally.

Stanley, in particular, needs to be less of a baritone and more of an animal (though I approve of the way Previn has the saxophone echoing his howls of "S-T-E-L-L-A" at the close of Act I). Rodney Gilfry looks lean and hunky with or without his shirt, but he's never really dangerous, just as Renée Fleming's voluptuous Blanche is never really unhinged - until her "mad scene", that is. Every opera should have one of those.

Fleming is, in a sense, too much of a good thing. Blanche DuBois is a huge role, and her vain belief that "soft people have got to shimmer and glow" is taken to extremes by lines that gives her every opportunity to do just that. But it's a little unremitting, this voluptuousness. Fleming's best scene in the opera is the scene with Mitch (the excellent Anthony Dean Griffey) which culminates in a hair-raising scena where she recalls the tragedy of her gay lover. Here, at last, the stakes are so high that there is real dramatic colour in Fleming's singing.

In Brad Dalton's rudimentary staging, a pretty young man in a white suit - the gay ghost - is an eerie presence throughout. As the immortal line, "Whoever you are, I have always depended on the kindness of strangers" hangs in the air, Previn and his librettist have Blanche take her leave of the real world with a haunting repetition of the words "whoever you are" directed at each of the protagonists in turn. At the last, she takes the ghostly young man, the love of her life who proved so elusive, by the hand. Hell, I'm not sure that's even better than the original.