A perfect evening in the Puccini Festival in Torre del Lago: Renée Fleming, presented to the crowd as New York's Queen of the Metropolitan Opera, is resplendent under the rising moon, singing an open-air gala concert in four languages. But the next morning, this corner of Tuscany becomes Torrents del Lago beneath a violent thunderstorm; heading out to interview the American diva, I'm up to my shins in floodwater. It feels gloriously operatic.
Now a new aria awaits Fleming: Arne's Rule, Britannia! On 11 September, she will be the star soprano soloist at the Last Night of the Proms. She's an apt choice for the occasion: she, perhaps more than any other diva, can sing anything she likes and, in seconds, have an audience eating out of her hand. It will be just the latest in an ongoing stream of new experiences for Fleming, whose reach extends well beyond her operatic home turf to some very unexpected corners indeed.
Tony Palmer's superb film portrait of her opens with footage showing her singing with the Muppets; she was the inspiration for the singer in Ann Patchett's novel Bel Canto; and she's written a book of her own, The Inner Voice, a revealing portrait of her professional life. Her golden-haired, green-eyed image features prominently in advertisements for Rolex and a perfume, La Voce by Renée Fleming, proceeds from which go to the Metropolitan Opera House. And, like the legendary soprano Dame Nellie Melba, she has inspired a dessert: the chef Daniel Boulud named "la diva Renée" after her. It is laden with chocolate and adorned with a musical emblem from Der Rosenkavalier.
She has detractors, of course; her ventures into numbers from musicals, jazz and, most recently, rock have sent some purists into meltdown. But she's still opera's golden girl, and her voice, with its distinctive, creamy gorgeousness, its emotional directness and honesty, proves why.
Fleming, 51, was born in Pennsylvania into a family in which, she says, "it was taken for granted that we all performed". Both her parents were music teachers and her first musical inclination was composition. It was not until years later that her voice truly began to shine. "Recently I came across some recordings of myself as a student aged 18 or 19 and I sounded like an insect," she laughs. "Five years later I was starting to sound like myself." Fleming was 29 when her major breakthrough arrived, with a win at the Metropolitan Opera's auditions.
A slow, solid beginning was vital, she suggests: "My teachers were very concerned to allow my voice to develop naturally. They didn't push it. They made sure that I wasn't trying to produce a more mature sound than was appropriate for my age. I owe some of my longevity to that. All these issues are so important: what repertoire you sing, the weight of the sound, the range of sound, how one sings – there's a staggering amount of potential pitfalls. It's a miracle that anybody manages to sing at all."
On a performance day she worries as much about her state of mind as about the voice. "I like to stay quiet and be alone," she says. "It's about the mental focus required for a performance. If I have to sing one aria at a dinner, to me it's the same as a three-hour opera. If you've ever had stage-fright, you'll know that performing requires mental preparation to guard against that."
A singer's voice changes all the time, and hers is no exception. "If you've established a solid technique, then you need to maintain it every day: there are constant micro-adjustments," she insists. "Every single day is different: your muscles' stretch, your stress levels, the air, the irritants. It's so challenging to perform with an instrument that's part of your body. Our careers are very short-lived – 20 years go by in a flash and suddenly you're thinking, 'wow, I don't know how much longer I have'. Everything now is a gift."
Fleming's quiet poise and straightforwardness – most un-diva-like – almost belie her propensity for tackling projects new and strange. She recently startled fans, critics and her two teenage daughters by recording a rock album, Dark Hope, featuring covers of songs by the likes of Tears for Fears and Peter Gabriel. "It was such a far-out idea and the timing was so perfect that I felt the stars had aligned and I just jumped in," she says. "I don't ever want to let a sense of stagnation set in – I'm always trying to learn something new. And I think it's vital that classical music should be less cut off from the rest of the world than it currently is."
She was startled by the demands of the music, though: "The total lack of stylistic freedom and the amount of discipline – it had much more in common with Mozart than the blues. It turned my assumptions about rock music completely upside down." The initial response from her daughters, Amelia and Sage, was "not good!" But she roped them into the album's backing vocals. "Sage wants to be a pop singer and I thought it would be a chance for her to see whether it can work," Fleming admits. "Maybe she'll change her mind..." Fleming has been divorced from the girls' father, the actor Rick Ross, since 2000.
Her next venture is strictly classical, though unusual in its own way: a concert programme focusing on Vienna at the turn of the 19th century, with songs by Zemlinsky, Schreker, Schoenberg and Korngold. "I haven't sung recitals for a couple of years and I'm desperate to get back to it," she declares. "This programme has me extremely fired up." She adores Korngold and sang some of his operatic arias at the Proms several years ago. "I'd have loved to do the whole opera Das Wunder der Heliane," she says, regretfully. "The vocal writing suits me perfectly, but the orchestration is just too heavy for my voice."
Her operatic choices are slowly contracting. She still sings around ten lyric soprano roles, among them the Marschallin in Strauss's Der Rosenkavalier and the Countess in his Capriccio. But she says she has "finished" with La Traviata, Manon and Thaïs. Is her voice changing? "Not really – but I am, in myself," she says. "I feel perhaps less resilient; and I don't need to climb every mountain every day as I did for so long."
The Last Night of the Proms may look like quite a mountain to climb, but its nature had passed her by. "I've sung at the Proms a few times and I wish every city could reproduce it because it's so fantastic," she says. "But then everyone said, 'you must do the Last Night,' and I thought, 'what's the big deal?' So I went onto YouTube..."
At last she realised what she's in for: "It's a carnival! I'm looking forward to experiencing it first-hand." Besides the traditional numbers, she will treat the audience to a selection of Smetana, Dvorák and some of the Strauss lieder that suit her voice so perfectly. Never mind Britannia: Renée will rule the airwaves.
Last Night of the Proms, 11 September, Royal Albert Hall (0845 401 5040; bbc.co.uk/proms)