There’s nothing quite like the opening of Schubert’s Fantasie in C for violin and piano.

The piano shimmers pianissimo in hemidemisemiquavers, and a long, yearning violin melody emerges – still pianissimo – as if from the mists.

A violinist’s role is to play it, not describe it, and Viktoria Mullova initially struggles to find the precise words.

‘I never make a story out of a piece: for me, the music is the music,’ she says. ‘It’s something you can’t describe except by playing it. What I can say about the beginning is that it’s like carrying something very, very precious? The most precious thing in the world, something which you treasure. It’s the way people feel when they say they speak to God. How can I put it? It’s something you love.’

And that’s before we’ve even reached the end of the first page. As we speak (on Zoom), Mullova is at her home in London.

The last few months have been light on concerts (Covid has seen to that), but she’s used the time to record music by Schubert for violin and keyboard, with Alasdair Beatson on fortepiano, and now the pair are preparing to perform the Fantasie live in Cambridge.

‘Gut strings are so thick that it’s very hard to hold the notes down, so I decided to go with the natural sound: to do the whole passage with a slightly whistly tone’

‘It’s a very famous beginning, and one of the most beautiful things written for violin and piano,’ she says.

‘It’s as difficult for a pianist to achieve the sound of that tremolo as it is for a violinist. I’ve played it with some pianists who’ve said that it’s Schubert’s most difficult piece for the piano, full stop. They are terrified of it. And for the violin, it’s also very, very hard because it’s so slow and the beginning is coming from absolutely nothing. And then also, the phrase is so long. Even the slightest movement which breaks this line is noticeable, because you are so exposed. Then on top of that, you have to make magic with it.’

With Mullova you almost take the magic as read.

But there’s one fundamental difference between Mullova’s 2005 recording of the Fantasie and her latest interpretation: Beatson plays a fortepiano by Paul McNulty – a modern reproduction of an instrument made in 1819 by Graf of Vienna.

And Mullova has put aside her Stradivari to play a gut-strung 1750 Guadagnini that she’s set up specifically for performing music of the Baroque, Classical and early Romantic eras.

That creates its own set of challenges – though it also presents solutions.

‘Musically, everything changes,’ she says. ‘I have to change the vibrato, I have to change the pressure of the bow on the strings, and the amount of the bow I’m using. And sometimes, when something dramatic is happening in the score and I have to make a big crescendo, for example – on the Strad I would automatically go with bigger strokes and more pressure, but I can’t do that on gut strings: they’ll start to squeak, and it’ll be a very scratchy sound. So everything’s smaller. It’s more pressure on the string, but less bow.’

We turn to the work’s second section, the Allegretto in A minor.

‘The Allegretto is also very difficult because most of it is piano and pianissimo and yet every note has to be pronounced. And as soon as you start to pay attention to that – to pronounce it – it becomes heavier and less light, which musically doesn’t work. To achieve the right result, you need to have lightness and crispness – a sound that’s full of air. Plus you have to make music after all that.’

She continues: ‘When I began to play with gut strings, I started to play chords in a different way. With gut strings and a classical bow – not a baroque bow, a classical bow – you can’t play three notes together. They have to be played arpeggiato.’

She points to bar 124. ‘A long time ago, I played those chords quite aggressively, but now with gut strings, it’s absolutely out of the question. And also, musically, it works [to play more gently]. It’s very nice because it’s more playful when at the end of the phrase suddenly you have this very light arpeggiato chord. It changes the atmosphere.’

As with any great chamber work, however, interpretation is a partnership.

Naturally, Mullova is best placed to tell the violinist’s side of the story, but as the piano takes us into A flat major for Schubert’s central Andantino, she can’t help but praise the artistry of her duo partner.

‘I must say, by the time I start playing in this Andantino, I’m already so inspired by what I’ve heard. When I play with Alasdair I play better, for sure: he’s just such an inspiration. Only yesterday, we had a rehearsal, because we are giving a concert in a week’s time, and I was surprised by how much better I played than when I’m practising alone at home.’

This is just as well, because the sequence of variations that follows presents some of the gnarliest technical challenges in an already formidable work.

‘Gut strings are so thick that it’s very hard to hold the notes down,’ she says, referring to the eye-watering demisemiquaver violin embroidery in the third variation. ‘Even without gut strings, it’s already really, really hard. It’s technically almost impossible to play. So I decided to go with the natural sound: to do the whole thing with a slightly whistly tone, almost as if I’m playing sul ponticello. It’s ghostlike – I wanted to find something interesting as a colour.’

And of course, that sound complements the glinting brilliance of Schubert’s piano-writing. Still, it must be a relief to land on the C major Allegro vivace that launches the Fantasie’s finale?

It turns out that we’re anything but home and dry: ‘Schubert didn’t play the violin,’ says Mullova.

‘There are some bits that are almost unplayable. I can give you an example: bar 529.’ She indicates a passage of string-crossing sextuplets.

‘I’m absolutely sure that he wouldn’t have written that if he had been a violinist.’ But what of the energy of this final sequence – the sunlit, C major euphoria? I suggest that the final bars are almost ecstatic in their effect, and Mullova agrees – but with qualifications. It takes a lot of effort to make music like this sound effortless. ‘I’m not ecstatic, at all – when I’m playing this, I’m just hoping and praying that I play all the notes and that they actually come out, because it’s very, very hard. So – ecstatic, yes. It should sound like it’s ecstatic, for sure. But it’s hard work too!’