Quite simply, he wasn't acting. If you watch the credits, it says 'Hand double for Georffery Rush: Himself' HE WAS PLAYING THE PIANO HIMSELF IN THE FLIGHT OF THE BUMBLEBEE SCENE and at all other times where you see Geoffery Rush playing. If you don't believe me, watch the scene again. Pianists will notice that he has perfect hand posture (his hands are held as if there was a bubble under the palms) and that he is playing the exact notes required for the piece.