One more thing:
If I’m playing an instrument-even one you can duet on (Piano, etc.)-I will get extremely irritated if you start playing on it while I’m playing without my proper consent. You usually do not know what I’m playing/don’t know how to play what I’m playing and will distract me with errors when I need to focus on my piece.
Wow, I didn’t realize that anyone thought that was ok.
Yeah, don’t ever play an instrument while someone else is playing it, unless they’ve given very clear consent.