There is much more to playing the clavier than playing written music. Do you realize with accompanying there is often nothing written out but the bass line–the left hand? There might be a few notations as to a suggested harmony, but it is up to me to fill in the music, at the proper volume, style, and harmony for the soloist–often instantly. I’ve heard it said that Bach questioned whether the soloist or the accompanist deserves the greatest glory.
― Nancy Moser, Mozart’s Sister