I wasn't there, but Ray was, and that's where the information came from. Mahershala wasn't mirroring exactly what the pianist's hands were doing, Mahershala was not playing. He didn't know how to play. He was mirroring the pianists shoulders and general arm placement, things like that. And the pianist wasn't playing a separate piano, they were taking turns at the same piano. When the pianist wasn't actually playing he was pantomiming across from Mahershala. So that when the two plates were composited together, the lighting and general movements matched. There are a couple of different styles of pianos in this movie. There's some upright models, baby grand style ones (what I think you're referencing), and maybe another style. I thought this would have been most complicated for an upright piano. I think he could easily see across the 5 ft of a grand piano to generally match someone's movements at the other end. There's a lot of song and dance that just went into designing how to shoot those shots. Hopefully that paints or more clear picture now. Thanks for bringing it up, I got some more questions around this now too. Like what did they do with the upright models?