I saw and heard him in rehearsal with the Victoria Symphony back in 1973. He was playing a Shostakovich cello concerto which had been written for him. Every few bars the conductor would stop and ask Rostropovich how it sounded until finally, he gave over the whole rehearsal to Rostropovich so he coud talk directly to the orchestra, rather than the conductor pointlessly relaying Rostropovich's comments.
I found myself almost being pulled off the seat (I was in the balcony) by Rostropovich's pianissimo passages. One of those very rare and valuable musical experiences.
