"Be not afraid. The isle is full of noises/ Sounds and sweet airs, that give delight and hurt not." Thus spake Caliban in The Tempest, but he might have been describing the Royal Opera House, where a gracefully Wildean Prospero - working in secret, but visible on every in-house monitor - is conjuring an opera from Shakespeare's play. On this enchanted isle, interlopers may hear a sudden outburst of brass, or woodwind singing sweetly, or a long caress on strings: the air is a kaleidoscope of shimmering orchestral sounds.
Thomas Adès is giving no interviews about the work, which he is still refining as he conducts rehearsals, and his director, Tom Cairns, and librettist, Meredith Oakes, are likewise keeping mum; the singers, too, are sworn to secrecy about the show. But when I quiz those singers, a little more than a week before curtain-up, they're pretty much in the dark themselves. As Philip Langridge, who is playing the King, puts it: "We're flying by the seat of our pants.
"But that isn't unheard of," he adds. "If you go back to Mozart, they weren't getting their parts until the night before. And we did get bits, as they rolled off the press at Christmas. There's no time for diva behaviour here. We're helping each other through it, as actors do." According to Toby Spence, who plays the lovelorn Ferdinand: "People were stressed about having to learn their parts so late. But Adès's music becomes its own mnemonic." They don't know how what orchestral underpinning they will get - they're working with a piano reduction - but they all speak with wonder at the vocal gymnastics required from Cyndia Sieden, whose Ariel is the musical heart of the piece. "When I first looked at her part on paper, I thought it wasn't possible," says Spence.
Sieden thought the same. "The tessitura [pitch-range] was higher than anything I'd ever done before," she says. "It was intimidating. I had to begin with a two-octave leap to high E, and there are 14 more high Es on the first page - I counted them. It doesn't feel normal. I asked Tom if he might take away some of the words, because mere sounds would be more 'do-able'. But he said no, that was exactly what he wanted. People had been telling me that by creating this role I would own it for life. But I thought it would be so vocally damaging, it could be the last thing I ever sang."
She did make Adès remove a couple of high Gs that she flatly refused to sing, but she is now doing the part, and by all accounts brilliantly. "It's like the four-minute mile," she says. "Something which seems unachievable, then someone does it, then everybody else says 'Hey, I can do that!' - and the bar is raised." This elfin soprano - perfect casting for Ariel - may be new to Covent Garden, but she's famed throughout Europe and America as a Queen of the Night with a preternaturally secure coloratura. Whence came this gift? Not from forcing in her Los Angeles childhood: "I didn't have a talent for a musical instrument, so nobody thought I could train as a singer. But I grew up singing folk songs with my sister - I sang all the time." She worked her way through college as a lounge-singer with a band, got nudged by her tutor towards opera, and began to win competitions. "The first was a local one in Seattle, where the prize was a thousand dollars. You had to sing three arias, so I began with the Queen of the Night. They had a limit of three minutes, and cut me off with an egg-timer. The second aria was their choice, so they asked me to sing it again - to see if I'd stumble. That was my introduction to the circus side of the business, the jumping through hoops." How did she feel? "Angry. But I got my thousand dollars."
She still had "no big ideas" about a singing career, but felt impelled to take lessons from the great Elisabeth Schwarzkopf for six hours a day, six days a week. "She was totally uninterested in my thoughts - I had to do it her way - but she enjoyed having someone she could sing through, someone who could approximate to what she wanted. It was all about the 'dramatic moment', and how you could colour it. If I'd been an older and more experienced singer, it wouldn't have worked - I wouldn't have bought it." But she did, and was then launched into a series of productions where her singular ability won her all the coloratura roles.
In The Tempest that coloratura will be stretched to the limit - indeed, beyond it. "The way Tom Adès wants it done, the words are more colour than anything understandable." So what will we hear? "I wish I knew. Part of the difficulty of it is that, as a singer, I don't hear much when I'm singing that high. And what I hear is not what other people hear. It's funny-sounding, quiet - more like a thought. But that's a thing that all singers have to learn. If we try and sing in a way that sounds good to us, it doesn't sound as good to other people. And the higher you go, the truer that is. Because then you're hearing more with the resonance in your bones than you are with your ears." She still worries about what the part may be doing to her vocal health: "I have to be careful not to sing it at pitch too often - at least, not more than once a day." How on earth does she rehearse? "By singing it two octaves lower!"
As for what we will make of it, Sieden is upbeat. "It's the absolute opposite to Broadway's junk food, but it's going to hold up well as a gripping experience. I've got friends and relatives coming who are not musicians, and I'm confident they're going to be thrilled. But it's not fair to judge it against Mozart." What does she mean by that? She fishes for words, then says half-jokingly: "It deserves the right to fail." And clearly there are risks involved. Meredith Oakes's libretto often hits a sweetly lyrical vein, but there are times (though perhaps this won't matter overly much if the words can't be made out) when she falls into Lloyd Webber mode. Moreover, our expectations are inevitably high, given that Shakespeare "scored" his play - asking at one point for "solemn and strange music" - and that even Caliban's language possesses a guttural melodiousness of its own.
Sieden does let slip one piece of classified information - that this island will have dinosaurs on it - but the main evidence of risk lies in the casting for that thing of darkness, Caliban. It seems that Adès wanted Ian Bostridge from the start, and Bostridge - tall, willowy, ethereal rather than earthy - is what he's got. When I ask that cerebral tenor how he sees the role, his normal articulacy all but deserts him. This Caliban, he muses, may "reflect ideas about the nature of sovereignty". Will he reflect notions - as has often been argued - about racist imperialism? "That's an idea which has had its day - and casting me is a pretty strong indication of that." After a modest disclaimer about being in any way special, he offers a startling thought: "Vocally, one is what one is. My casting seems a re-emphasis of Caliban's humanity - of the idea of him being a dispossessed noble person. I'm used to playing princes, and one of my friends suggested I should play him as a prince." As for body language, Bostridge says he's currently limping as a result of a foot injury, and that limp may survive as part of the performance. Tricked out with dinosaurs, and with Caliban as a limping prince, this will be a Tempest unlike any we're seen before. Will it work as drama? Who knows? But the beauty of its music is surely not in doubt.
'The Tempest', Royal Opera House, London WC2 (020-7304 4000; www.royalopera.org) from 10 February