Nancy Moser said this quote

“ 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 wether the soloist or the accompanist deserves the greatest glory. ”