Playing with the choir often involves quite a bit of coordination between organist and conductor. You have to work out with the conductor the registration, tempos, and even touch.
The thing about playing with Rasa Gelgotiene, the conductor of Vilnius University choir "Pro Musica" is that she allows total freedom of expression from the organist part.
Maybe she's this way only with experienced organists, people whom she trusts that she leaves the organ introduction without any conducting. I was also very free to choose registration I wanted, provided the balance between the choir, soloists, and organ was at the optimum level.
Of course we have to keep in mind that Rasa's relaxation comes partly from her enormous lifelong experience of working with choirs and from the fact that she has two fantastic assistants whom she trusts - Ignas Garla and Modestas Jankunas who worked this piece (the Solemn Mass for Saint Cecilia by Charles Gounod, 1855) with the choir over the summer.
So even though this organ is as complex as any mechanical instrument can get (especially with the ventil system), I actually had a pleasure and a relaxed feeling while playing with them.
The soloists (soprano with heavenly high register Gunta Gelgote, tenor with dramatic color Viesturs Jansons, bass with a special interest to early music Nerijus Masevicius) were also fun to work with.
I even almost didn't have to look at the mirror (partly because the mirror was located very high and partly because WE FELT EACH OTHER.
I think this is the key.
If you want to stay relaxed in these seemingly unpredictable and stressful situations (and believe me, we had plenty that day), you have to feel the performers, the music, and the instrument.
Don't worry - it comes with experience (I had my doze of stressful but useful appearances as an accompanist and chamber musician earlier in my organist career).
I think the festival uses the services of an organ builder from another city who maintains and tunes the instrument during the festival so everything worked well (more or less - it's a really LARGE organ - there's always a room for some ciphers and other adventures which is part of the deal).
A great assistant who stays invisible to you helps a lot, too - thanks to Maija Zakis who operated the ventils and page turns for me. She also turned on the Barker machine very quickly during one episode of the Gloria while I was wondering why the 2nd manual was so out of tune and worked only half way.
And of course all of this wouldn't have happened without the tremendous efforts of Dace Bluke, the director of Via Cultura and her team who organized this 14th International Organ Music Festival in Liepaja, Latvia and which continues to keep this incredible organ a symbol for the entire city.