If you met Sally Matthews in the street you might not guess that she is one of Britain's finest sopranos. Quiet, serious and rather reserved, the 38-year-old singer is anything but an obvious star; but on stage her voice speaks for itself. Blessed with great range and a rich tone containing unusual warmth, colour and shadow, her refulgent yet pure sound is ideal for Mozart, Strauss and, not least, French music.
Matthews is about to take the leading role in Francis Poulenc's opera Dialogues des Carmélites at the Royal Opera House, amid an all-star cast conducted by Simon Rattle. Operatic success does not get much bigger than this, but she refuses to play the diva. To her, opera is teamwork; and she prefers to avoid repertoire like the more melodramatic moments of Puccini, which possibly attract a different type of personality. "Sometimes the big egos completely detract from what we're doing," she muses. "I've worked with a few of them and I didn't like it much. It should be all about the music."
The Southampton-born singer's career was launched when she won the Kathleen Ferrier Singing Competition in 1999, but it was a special opportunity at the Royal Opera House in 2001 that subsequently determined her direction. She jumped in as Nanetta in Verdi's Falstaff, replacing an ailing colleague – at such short notice that she arrived for the dress rehearsal expecting only to watch: "Suddenly I found myself being bundled into costume and told 'You're on'. And you've just got to do it."
There she attracted the attention of the production's conductor, Bernard Haitink, who gave her valuable advice. "He said that everyone would offer me the world – and I should take things slowly and steadily. Therefore, I've always erred on the side of caution and known when to say no to a role. You have to take care of your voice if you want to last."
Matthews paces herself rigorously, factoring in plenty of time at home with her daughters and her husband, the Icelandic tenor Finnur Bjarnason. She was recruited for Dialogues des Carmélites only in October – replacing Mrs Rattle, the mezzo-soprano Magdalena Kozena, who is expecting a baby in June.
She clearly adores working with Rattle. "He is very relaxed," she says. "There's lots of energy, but no anxiety." A few weeks ago she stepped in, again at short notice, to sing Haydn's oratorio The Creation with him and the Orchestra of the Age of Enlightenment: "That was wonderful during the early rehearsals for the Poulenc," she says, "because on the concert platform, unlike the opera house, we stand close together and can be almost telepathic in the way we think about the music. It's nice to be reminded of what that connection is like."
A strong rapport is essential in the finely wrought drama of Dialogues des Carmélites. Set during the French Revolution, it deals with faith, fear and courage. Matthews plays Blanche, a young nun who battles her own sense of cowardice, but ultimately chooses martyrdom at the guillotine together with her holy sisters. "She's a complex, insecure, frightened character," Matthews says, "but underneath it there's immense strength."
The mass execution is musically graphic, the repeated zing of the guillotine's blade woven into the score. "That final scene should be devastating," says Matthews, "but it can also be incredibly uplifting. You often see people crying in the audience – many out of sadness, but perhaps also because it's such an amazing thing for a group of women to do, giving their lives to show their faith. They're just not afraid of dying." The director, Robert Carsen, has chosen a symbolic, choreographic way to behead his cast. "It's one of the most beautiful pieces of staging I've ever seen," Matthews declares.
Guillotines aside, Matthews is one soprano who definitely keeps her head when all around are losing theirs. "I love being on stage," she remarks. "But afterwards I'm happy to slip out the back and quietly go home."
'Dialogues des Carmélites', Royal Opera House, London WC2 (020 7304 4000) 29 May to 11 June