Share 'Session Etiquette'

Stop me if I've asked this before. Should you ever just start playing? Should you sit at the bar, fiddle case obviously in hand, but wait to be asked? Does it depend on the session? Any special tips for bodhran players? What goes through your head when they seem to insist on playing everything you play, needed or not? Special tips for singers? (Like, if all I know is rebel tunes or drinking s...