interesting disagreements in the past between pianists and conductors as to who is "in charge"
Isn't there a story about a pianist and conductor, where the conductor before the performance made an announcement that the "visions" of him and the pianist were so different they "agreed to disagree", so the audience was 'warned'. Correct me if I'm wrong, but I think it was Beethoven 5, as fought between Bernstein en Gould.
An analogous relationship should help you answer the question: Who runs the piano in a violin show-piece (e.g., Kreisler)? Mike