Opera review: Cosi fan tutte Glyndebourne : A musical course in the rules of love

Glyndebourne's first audience of the season must have wondered if they'd walked in on a rehearsal. In a sense, they had. Excuse me? Well, the set (by Richard Hudson) looked finished enough: a large empty room - industrial iron pillars, radiators, a couple of plastic chairs. Anybody who's ever seen the inside of a rehearsal studio will get the picture. But who was this casually dressed man ambling in so distractingly during the prelude? Not Graham Vick, the director, surely. An associate? In a sense.

His name is Don Alfonso. And he's got some wild ideas for this show. His two leading men, Ferrando and Guglielmo, have arrived, too, scripts in hand. They're pretty incredulous about the plotting. It'll never work. Give it time, says Don Alfonso. A few more games, an improvisation or two, some subtle application of "the method", and we'll be in good shape. You'd better believe him. He's the director, and directors never lie, do they? Stage management (they're the ones in the headsets) are now in place, props at the ready. Cue.

Cue Mozart's Cosi fan tutte - the School for Lovers. Note the subtitle. Suddenly it all makes sense. We know what Graham Vick is up to. This Mozart/Da Ponte masterpiece (and doesn't it only get better with familiarity?) is all about youth and learning and practice makes perfect - or even imperfect. It's all about those final rehearsals for adulthood. And the mistakes that you carry with you into the performance if you're not awfully careful. This is the conceit (or is that deceit?) of Vick's devilishly clever staging, and he and his wonderfully vital, young Glyndebourne cast carry it off brilliantly. The cleverness lies in the way that the boundaries are blurred here: where does the rehearsal process end and the play - or opera - begin? How much of themselves do the performers bring into the rehearsal room? "Is this for real?", says Guglielmo at one point. Well, is it?

Naturally, the rehearsal context means that however preposterous the plotting gets, however extravagant the behaviour of our main protagonists, you can always blame the process. Actors will be actors. Which makes all the bluff and double-bluff, the wooing and the resistance, as silly and as funny as rehearsal-room games can make it. Anything goes. Try it. Interesting relationships are established here with tables and chairs. And put a couple of actors in Arabic gear (all that fabric to play with) and the possibilities don't even bear thinking about. Vick even brings his chorus into play: they are the rolling waters of the Bay of Naples, they are a camel train come to bear the sisters to and from their suitors. And modern dress has its advantages, too. Levi-Strauss could make capital of the moment where Doctor Despina's mighty magnet draws the young men's pelvises off the floor in a gesture that is as funny as it is overtly sexual.

But this same director can back-light the great trio of act one so that only the voices beg our attention with their tearful benediction. And they are fine and true voices. Barbara Frittoli (Fiordiligi) and Katarina Karneus (Dorabella) are vocally, temperamentally, beautifully aligned, the blend and oneness of their timbres ideally conveying Mozart's sisterly symbiosis. So, too, the pairing of Roberto Sacca (Ferrando) and Natale de Carolis (Guglielmo), the former an engaging lyric tenor (lovely, truthful account of the aria "Un aura amorosa") but a voice with backbone and attitude, too. Alan Opie (Alfonso) more than served his function as the all-seeing, all-knowing instigator of the exercise, conniving, bullying with charm, while Daniela Mazzucato's Despina was the waspish voice of scepticism. Together, as an ensemble, they made something really pointed of the frantic act one finale, its blistering coloratura going off like a freshly shaken, freshly popped bottle of bubbly. Andrew Davis and the London Philharmonic were a robust alliance in all of this, exquisitely counterbalancing the diaphanous femininity and rude masculinity of the great score.

But the opera finally turns on the role of Fiordiligi, and in it, Barbara Frittoli fulfilled much of the promise she's been displaying of late. This is a singer for whom the spirit is always the prime mover. She began the preposterously thrilling (and difficult) "Come scoglio" seated firmly in her resolve. But as the contortional leaps and bounds of the vocal line intensified her protest, she rose to it (in every sense), saving the biggest notes of all literally to scare off her advancing male predator. "Per pieta", by contrast, was the heart and soul of the opera, now contrite, now remorseful, now steadfast, dropping precipitously from high soprano to contralto and back again. Suddenly, Fiordiligi was no longer just a character in an opera, but a real young woman with real concerns.

So when, finally, the cruel deception is over and she, along with her sister, graduates from this "school for lovers", what she has learned will have changed her forever. It's only a split second, but the look that passes between our reunited couples in the moment before they go their separate ways once again gives them, and us, pause for thought.