Opera: Cecilia Bartoli: Cecilia, you're breaking my heart...

Why would a star making her Covent Garden debut need tea and sympathy? Anna Picard meets Cecilia Bartoli

After shifting our appointment some five times in seven days, I wasn't expecting to like Cecilia Bartoli. Not that changing arrangements is unusual in opera, oh no, but Bartoli's elusiveness is legendary and it's there between the lines of every article written on her. Here's my favourite: a 1999 interview that sees Bartoli cooing over pictures of the unfortunate journalist's bambino – whose first birthday party he was missing in order to meet her. Hmm. Add to this rumours of a recent hissy fit with Gramophone magazine, and her minders' claims that she was "really sweet" seemed less than credible. Surely no-one gets a career like Bartoli's by being "sweet"?

First the facts. Cecilia Bartoli is 35 and has been singing professionally for 15 years. She lives with her mother, Silvana, who is also her teacher. Her most populist role is Rossini's Cenerentola – the perfect vehicle for her small but magical mezzo voice, but hardly Tosca – yet she outsells every living opera singer except Pavarotti, without resorting to crossover or football stadium appearances. She has turned down even Carmen many times, choosing instead to explore lesser-known 18th-century works with original instruments as her preferred accompaniment. It should have been a recipe for disaster in today's market-driven music world but Bartoli's career has broken the mould. Which is why she is powerful enough to be able to choose an obscure Haydn opera, L'Anima del Filosofo, for her long-awaited Covent Garden debut. Which is why she is talking to me.

When the world's favourite mezzo finally arrives, I'm astounded. I'd expected a haughty dollop of corseted glamour, a Lollobrigida or Loren, but Bartoli – who wears a studenty ensemble of T-shirt, fleece and ancient jeans – is Ab Fab's Saffy. A la Romana. For all the smoke and mirrors of her publicity portraits, in the flesh she's the girl next door: shy, short, curvy – as she ruefully points out, it isn't just her voice that has grown over the last few years – with an eager childlike smile, and hair that does that tumbling thing that only fictional or Mediterranean hair can do. If most divas look like millionaire lady novelists – the kind whose books are sold with bas-relief pastel covers – Bartoli looks like their typical customer. Was it her charm that won me over? Hell, no – most opera singers can turn on the charm when they want to. It was the fact that she blushed like a schoolgirl when she admitted enjoying the process of researching old manuscripts. Guess what? Cecilia is a swot!

Bartoli may be as protective of her instrument as any prima donna ("I do not scream the day before I sing..." she breathes shooshily) but she's remarkably uninterested in talking about it. Instead she discusses the Orphic myth, and the link between creativity and the will to survive that Genio, one of her two roles in L'Anima, embodies. We also cover the Ancient Roman theory of genius, and the issue of historical pitch (she asked for A=430 – a quarter-tone below modern concert pitch – but was turned down by Covent Garden). And lest she sounds obsessive about music, it was Metastasio's libretti that brought her from Vivaldi to Gluck – whose Italian arias form her latest recording – and on to Schubert. So when Bartoli says she likes "to have a sense of chronology", you'd best believe her. The eminences grises of opera may have thought she was barmy at the start but the bright, thoughtful woman I'm talking to would have been bored stupid by a life of "stand and deliver" singing and standard repertoire.

If we get a little linguistically tangled from time to time it's not during the parts of the conversation that she wants to have. The only hints of steel behind the softness are a sharp moué of disapproval when I raise her spat at the Met with Jonathan Miller ("Phhh! Prehistoria!"), a rather heated denial of any knowledge about the upcoming Gramophone Awards, and a slight chilliness when I enthusiastically refer to her latest CD as marking a new era for her voice. (She reckons her Vivaldi disc did.) But being surrounded by the hysteria of instant stardom is bound to have an effect and despite her cheerfulness, there's something sad behind her eyes. Bartoli didn't get the find-your-own-way freedom and fracas of normal early adulthood. She didn't get the flat-shares, or the overdrafts. She got the Met and Salzburg.

"At the time it was fun," she says. "The only problem was that I was not with my friends. I mean, I was 20 and I was travelling with my mother but this was hard – to leave my town and my friends and perform in all these different places." Did she make new friends? "Mmmhhhh ... Let's say yes. But new friends and you have a career? It's not that genuine, it's not like friends from before. They look [at] you in a different way." Are you lonely, I ask. "Yes," she says, with no further explanation. Oh come on, I want to say, you're not Julia Roberts! Mitsuoko Uchida goes to the South Bank all the time and people don't pester her, they just smile shyly. Would it really be so different?

Well, maybe it would. As I leave the Opera House, two stage hands are taking a fag break. "Is that X?", asks one, naming a very famous tenor and gesturing to a slumbering Bow Street tramp. "No, it's Y," says the other, naming an even more famous soprano. "She doesn't like to be recognised off-stage." Will Cecilia be the butt of that joke next time? I hope not. I liked her. But I wish she could let that lovely hair down and realise that not everybody outside the Roman suburb where she grew up is going to fall over speechless when they recognise her. And if she ever fancies putting her feet up in front of a Danielle Steele mini-series, I'll put the kettle on for her. After all, it's only opera.

'L'Anima del Filosofo': Royal Opera House, London WC2 (020 7304 4000), from tomorrow to 31 October