The seat is sacred
Rule number one across every parlour in Japan and here: a player at a machine has the right to their seat and their trays. Don't hover behind someone mid-session, don't lean on the machine next to them and never — ever — touch their ball trays.
If a machine you want is taken, ask the host desk to flag you when it frees up. Regulars do this constantly; it's completely normal. What's not normal is standing behind someone's chair like a storm cloud.