Nick Kimberley thrills to a close encounter with the Hungarian mezzo-soprano Vesselina Kasarova
It's not always a good idea to observe a singer too closely. The voice is supposed to be natural, relaxed, but the grimaces and scowls necessary for perfectly produced sounds seem at odds with the effects they engender. On the other hand, it's good to be reminded that singing is muscular labour, a fact obscured by talk of "honeyed sounds" and "creamy legato". To appreciate the physical and intellectual work is to become more deeply involved.

Nevertheless press seats at concerts are usually far from the stage, as if journalistic "objectivity" requires an emotional buffer zone. So I was surprised last month to be in the front row, within feet of the platform, for Vesselina Kasarova's performance with the Dutch Radio Symfonic Orkest in Utrecht. Face-to-face confrontation with the Bulgarian mezzo- soprano proved a thrilling experience.

For a start, Kasarova looks sensational, if not conventionally glamorous (although her stylish dress presents a decolletage that, for a classical singer, is rather bold). She is the kind of jolie laide gamine that French film directors went soppy about in the 1960s, something between a martyred saint and Jean Seberg street-selling newspapers in A Bout de Souffle. And when Kasarova sings, she's yet more bewitching. Her performance mixes arias from her RCA Victor recordings and pieces soon to be recorded, all of it, bar one Mozart concert aria, operative in origin. Singing opera in concert is a tricky proposition, and most performers opt for poise and restraint. Kasarova doesn't lack those qualities, but nor does she hold back from dramatising the music. To be sure, she doesn't roam around the way she might on stage, but she uses her body, and particularly her face, in ways few singers dare. Her hands shape the vocal line, and in "Che faro" from Gluck's Orfeo ed Euridice, her eyes mirror the agitation in the music. At times her responses lead into risky areas: during the orchestral introduction to "Une voce poco fa" from Rossini's Il Barbiere di Siviglia, her eyebrows twitch in time with the music. Comic overstatement or involuntary tic, mark of intense involvement?

She's hardly less bold in the way she uses her voice. In the Rossini aria, the drama hinges on the word "ma" ("but") that links the two facets of the character of Rosina: "on the one hand, I'm sweet and respectful ... but cross me and I become a viper." Kasarova turns that "ma" into an extraordinary buzzing, like a swarm of angry bees pursuing their tormentor. It's a remarkable sound, almost unmusical were it not for the intensely dramatic impulse behind such fearless vocal gestures.

Not that she turns bel canto into expressionist drama. Kasarova is a wonderfully idiomatic singer, able to locate the emotional heart of fiendishly difficult music, whether the filigreed decorations of Handel or the gorgeously extended lines of Massenet and Bellini. Her coloratura is not gratuitous display but a profoundly expressive language, and her physical presence is a natural extension of her vocal apparatus.

Now 32, she made her debut in this country in 1993, singing Rosina in a handful of performances at Covent Garden. Audiences here won't have another chance to see her in opera until the year 2000, when she sings Sesto in Mozart's La Clemenza di Tito at the reopened Covent Garden, although next week at the Wigmore Hall she shows another facet of her character in lieder by Schubert, Brahms and Schumann.

The day after the Utrecht performance, I interview her in her hotel. Her desire to communicate is no less intense, despite the fact that my Bulgarian seems to have deserted me, so, with an interpreter's help, we do the interview in German. She directs her responses to the interpreter, who stuggles with Kasarova's verbal torrent. Then while her answers are translated, she stares intently at me, willing me to understand.

"At first I studied as a concert pianist, but I'd always tried to sing a little myself, and I'd accompanied other singers. By observing them, I realised that singing is an expression of your innermost feelings, it brings out what's inside. Of course, playing the piano was also about feeling, but singing is more 'innerlich'. Playing an instrument, you can hide behind the fingers, behind the notes, whereas singing comes out of your body."

For a singer, uniting soul and body demands both meticulous preparation and mercurial intuition: "I don't want to take 'my' Rosina with me, to do the same things every time I sing a role. That's why it's important to work with good stage directors, to discuss things, to bounce my ideas off theirs. Sometimes you get directors who have no ideas, and I've sung in productions that I thought were simply too old-fashioned, too static. Being on stage is like playing a game or being in a film, and I love that. When I graduated in Sofia in 1989, I joined Zurich Opera and had the chance to work with famous singers. It was like going to school, not because I could copy them, but because I realised that good singers don't simply sing, there's something else that they're doing, something extraordinary, individual. That's what I try to find within myself when I sing."

Perhaps the fact that she is a mezzo-soprano helps. Many soprano roles might charitably be called drippy. A mezzo is made of sterner stuff: "When I was studying, I had a very good high register and a very good lower register, but there was a hole in between. I was told I was a dramatic soprano, but I was convinced I was a mezzo-soprano because when I sing for a long time at the top of the soprano register, it's not comfortable for the voice, the colours aren't there.

"I'm happy with my repertoire; one should stick to what one is good at. I would like to sing Mozart and Rossini for as long as possible, and, of course, certain bel canto roles, but I don't want to use my voice as an experiment. Enjoy the voice as it is, then see where it takes you."

Onwards and upwards is where it's taking Kasarova. We live in something of a golden age for mezzo-soprano voices, but here is a genuine individual, daring, occasionally even wayward, but always intent on communication. And, as the standing ovation she received in Utrecht demonstrated, you don't have to be in the front row to get the point.

'Vesselina Kasarova: A Portrait' and 'Mozart Arias' are available on RCA Gold Seal. Vesselina Kasarova sings Schubert, Brahms and Schumann at the Wigmore Hall, on Tuesday 12 May (0171-935 214).