Since this was the conductor Andrew Davis's first rehearsal with orchestra and singers, the Glyndebourne press department had decreed that, once it had begun, journalists were most definitely verboten. Von Otter, however, enjoys living dangerously - musically speaking - and wouldn't have minded at all.
She has been preparing the role of Beatrice since March. 'The process is very different from working on a recital, which has to be absolutely ready when you fly in for a performance. With opera, the initial work is just you and a pianist. The finishing touches can only be created when you rehearse with the orchestra and the whole cast. That's when all the colour is added. Some singers feel quite on edge with a first rehearsal, but for me it's really exciting.'
As far as roles go, Beatrice is, she says, 'a medium-sized role for me'. She loves singing Berlioz. 'Harmonically he is a little bit crazy, which I like. As a composer, he wasn't afraid to do the unexpected; he didn't try to get everything right. I can't explain why I enjoy it so much, but his musical language really speaks to me. Beatrice's aria is divine.'
For an opera singer, Von Otter does relatively little opera. Married to the Swedish actor and theatre director, Benny Fredriksson, she has two sons, Hjalmar, five, and Fabian, two. 'I was scared like hell that I would be on the shelf and had left it too late to marry and start a family. I always wanted children, but they completely change your life.'
For a couple of years after Fabian was born, she concentrated her work on just two opera houses, Covent Garden and the New York Met, plus concerts, recitals and recordings. The most un- prima-donna-ish of divas, Von Otter plans her schedule meticulously and works extremely hard in a profession not noted for making much in the way of allowances for the demands of motherhood.
Home is a beautiful five- roomed apartment near the centre of Stockholm. Not surprisingly, she has no interest in spending most of her life in hotel rooms, like so many opera singers. She prefers making short trips away from home, for recitals. 'Recitals are wonderfully satisfying. They make me feel my life isn't worthless, which is how it can feel sitting around an opera rehearsal for hours on end when all the producer wants to do is rehearse the chorus.'
This time, she's staying in London for 10 days, so the boys and their nanny are travelling with her. Her husband flies in later on. 'He's been incredibly busy for months, and today is the start of his holidays. It's very unusual for us all to be together somewhere when I'm working.'
In Sweden, school starts at seven years old, so at the moment Von Otter is free to come and go with her children. 'If I have the option between working in America or Europe, I'll always choose Europe, so I can get home quickly. If I'm away for a night and phone home, I ask if the boys have missed me and the answer is always 'No'. Although part of me wants them to miss me, the other part of me realises how lucky I am that I can carry on doing what I love, knowing that my children are happy. A few months ago I worked out what it all costs. It's an enormous amount, but it means our lives are much more comfortable. I think I've found just the right balance between work and family. I'm very privileged to stay home so much and still be in demand to work with some of the finest orchestras and conductors in the world.'
After a performance, she hates attending formal receptions or parties. 'When people from the Swedish Embassy ring to ask if they should arrange a post-concert reception for me, I always tell them, absolutely no. If the children are with me, I want to get back to them. If I'm by myself, I want to eat an enormous amount, sleep for hours, read magazines, and just enjoy my own space. Sometimes, music is going around in my head, which makes me toss and turn. It takes me ages to switch off. I couldn't imagine ever doing anything else but, as a mother, I wouldn't immediately recommend singing as a career. It makes enormous demands.'
Her favourite recital venue is the Wigmore Hall in London: 'The space and acoustics are perfect, and the audiences are so knowledgeable and receptive, which is a real joy for a singer.' But, apart from occasional concerts, she hardly ever performs in Sweden.
She is often asked what it is about the country that has given the world so many outstanding singers - from Jenny Lind and Jussi Bjorling, Birgit Nilsson and Elisabeth Soderstrom, through to today. 'There is a singing tradition in Sweden. We have literally thousands of choirs. People are used to projecting their voice properly and, like Italian, the Swedish language is good for singers. I have no idea where my voice and musical intelligence come from. I'm just very grateful to whoever sent it to me.'
Beatrice et Benedict will be sung in French, with spoken English narrations (written and performed by John Wells) in place of the original dialogue. Von Otter is grateful for the compromise. 'Singing in a foreign language is one thing, but this opera is full of dialogue and it would be quite difficult for me to make it sound dramatically natural.'
Back home in Stockholm, Von Otter devotes part of each day to rehearsing. 'I like to be thoroughly prepared for everything. I can't cope with stress and try not to do anything at the last minute.' For recitals, she rehearses at the home of her pianist, Bengt Forsberg. 'I go to his studio - for three good reasons. My piano at home isn't very good, my phone never stops ringing and the children always want to come in and play. It's very difficult for a singer who has a family to work at home unless you live in a big house, and I don't have anywhere to shut myself away.'
This July she will have a week in Provence with her husband - the first time they've been away on their own for ages. But before that there is almost a month with the Munich Opera doing Figaro and Der Rosenkavalier. In September, she gives an Edinburgh Festival recital with the fortepianist Melvyn Tan, then it's straight into a German tour and recording sessions with John Eliot Gardiner, doing a completely new kind of repertoire, songs by Kurt Weill. Then, in November, she's back on tour, in Monteverdi's Poppea, again with Gardiner, plus his Monteverdi Choir and Orchestra. There will be performances in Italy, Vienna and London.
'It isn't difficult to keep all the different songs in my head, but I need to wake them up. I've just looked at Der Rosenkavalier again, which I'm doing in three weeks. You can't just turn it on automatically. It's not like running water.' And with that, Andrew Davis called for the rehearsal to begin and I was ushered out into the pouring rain.
For Glyndebourne performance details, see opera listings, below