For a conductor who doesn't believe in stamping his own features on the score, Osmo Vanska is making quite a name for himself as an interpreter of his Finnish compatriot, Sibelius. It's all a matter of trusting the composer, he tells Stephen Johnson
Where were you on the night of 1 May? I was in Glasgow, listening to the cheers as the Scottish electorate sank Tory MP after Tory MP, and later to a group of men lurching across the River Kelvin singing "No more Tories in Scotland" to the tune of "Land of Hope and Glory". And before that - almost like a dramatic prelude to the night's event - came an exceptionally stirring performance of Sibelius's Fifth Symphony by the BBC Scottish Symphony Orchestra under its principal conductor, Osmo Vanska, who in his short stay in Britain has already acquired a reputation as a major, possibly "great" Sibelius interpreter.

The end of Sibelius's Fifth Symphony can all too easily be transformed into brash triumphalism. But there was nothing like that from Vanska; rather affirmation in a deeper sense: a musical heroism which could still stir, still convince in the despondent closing stages of the 20th century. Which made it all the more remarkable to meet the modest - or at least mostly modest - softly-spoken Finn who directed it.

Vanska becomes visibly uncomfortable at any language which glamorises the conductor. "I hate it when people speak about `Vanska's interpretation'. Who is Vanska? Who wrote the music? The composer's interpretation is what matters - in the score. If I can follow that, then this for me is the greatest pleasure. This is so close to my heart. If a writer has written a book, and then someone is reading the book, do we accept it if the reader is adding words or removing them? Does that make the original story better? I don't want to say I'm the only one who can do it - I'm not so stupid. I'm just saying we have to trust the composer. Not our own opinions."

Views like this were more widespread 20, even 10 years ago. Now orthodoxy leans more heavily towards the "interpreter". It's almost received wisdom to argue that you can spend too long with your nose in the score; it's not the letter that counts, but the spirit - and in the age of cultural dumbing-down, surely musical techno-speak is about as "elitist" as you can get? "I know these arguments, but how do you reach the spirit if not through the letter? Of course there must be freedom. Sibelius hated to give metronome marks for his music for that reason, but he gave them when he felt conductors got something wrong. What I wish is that people would sometimes just take what a composer wrote at face value and not give their view or the established view of tradition. Many conductors slow down in the final sections of Sibelius's Fourth and Sixth symphonies because... because that's what you do, isn't it? But try this music as Sibelius wrote it, with no slowing down, and different meanings emerge. Look at the end of No 6. This music is very personal. I think Sibelius wrote something that he wanted to be true in his own life but wasn't - a kind of transcendence, not nostalgia. But you must have some kind of inner identification with the truth of the music, or why do you do it?"

I'm sure most readers will agree with that last remark. Still, can that "inner identification" come solely through the score, or does it help to know something about the man who wrote it? Sibelius's diaries are full of wonderful remarks about the music, and about composing in general: "It's as though God the Father had thrown down the tiles of a mosaic," he tells us, "and it is my job to put them together again."

The glorious horn figure in the finale of the Fifth Symphony is described as the "Swan Hymn", and likened to a flock of swans vanishing towards the sun "like a glittering silver ribbon". Vanska leans forward across the table, eyes alight. "Exactly! It's all very helpful. Also the insights into his struggles and frustration. This is the paradox, though. If you were to imagine Sibelius the man from his symphonies, you would think of him as very strong, severe, purposeful. But in life he was full of weaknesses, failures - he was very sensitive. If someone wrote one negative word, he took it very hard. In the end we are talking about the music, not the man. And that you find in the score. When the orchestra and I did Beethoven's Fifth Symphony at the Proms, a critic wrote that it was like hearing a new piece. Wonderful? But I just followed what Beethoven wrote, and it then became alive."

Vanska and the BBC SSO's successes at the Proms - Beethoven's Fifth, another splendid Sibelius Fifth - are all the more striking because the Albert Hall's misty distances are radically different from anything the performers are used to in Glasgow: BBC studios or the intimate clarity of City Hall. "True, but the basic formula is the same. Still, I hate to do things planned. Every performance should be unique. I listen carefully to each performance as it happens. If a player does something different - a new idea about phrasing or colour, I have to go in with it - whether I like it or not! I must respond. I prefer this kind of performing art. If there is such a thing as `interpretation', it must not be like a CD - the same every time you play it." So directing an orchestra is more like participating in a chamber ensemble? "That's when it's best. And yet there are orchestras that need a strong father. That's hard for me."

It must be particularly hard in a work like Sibelius's First Symphony, which begins with an immense, partly unaccompanied clarinet solo -plenty of time for an unruly clarinettist to stamp his or her authority on a performance. "Aha... but I have a special relationship with that solo. In my days as an orchestral clarinettist I played it many times. I still remember my first concert as orchestral clarinettist in Turko - first half: Sibelius First, starts with solo clarinet; second half: Prokofiev Third Piano Concerto, starts with solo clarinet! For me, it's difficult to understand how you can conduct without that experience. Orchestral players have their own language - it's the language of the Kindergarten sometimes, but... well, how can you come from outside without knowing this and expect an orchestra to respond? That's one of the reasons I still bring my clarinet to rehearsal. In France, I once conducted the Sibelius First. There was a difficult little figure in the second movement, and they couldn't do it. So I went to the clarinet section and said, `Try it like this.' Of course, I had no problems after that."

Listeners outside Glasgow who missed Vanska and the BBC SSO's majestic Sibelius cycle when it was broadcast as part of Radio 3's Sounding the Century project can get some idea from a new CD in which Vanska and the Lahti Symphony Orchestra bring together the First Symphony and the toughest, but possibly the most rewarding of the symphonies, No 4 - a version which holds up well against any present competitor. And then there's Vanska's all-Sibelius Prom with the BBC SSO on 20 August: three magnificent but rarely heard works, the choral Kullervo symphony, the weirdly atmospheric Luonnotar, based on a Finnish creation myth, and a work that is virtually Vanska's own discovery, the long-unheard tone-poem, The Wood Nymph. Again, if you can't get to London, there's the live Radio 3 broadcast. "I'm proud of my CDs," says Vanska. "I hesitated for a long time before I felt I was ready to record these works. But ultimately the live performance is what counts - if there is such a thing as performing art, that's where it is"n

Osmo Vanska conducts the BBC Scottish Symphony Orchestra at the Proms: 18 Aug (Glinka, Tchaikovsky, McGuire, Shostakovich), 20 Aug (all Sibelius), Royal Albert Hall, London SW7 (0171-589 8212) and live on BBC Radio 3

Vanska's new recording with the Lahti SO of Sibelius Symphonies Nos 1 and 4 is on BIS (CD 861)