Like most GMs I run a game to entertain my players.  On occaision this also ends up being entertainment for other people floating around the room/apartment/store.  Most of the time it is gamers who for one reason or another have nothing to do and just want to listen in.  In some manner or other they wander/hover around the table listening and watching with minimal vocal input.   If they recognize the creature being fought or hear something that is just wierd the random squeak/laugh/cough will erupt.  Enough to get the table's attention, but not enough to completely distract the group.  I appreciate these, as it means I am doing my job and have made the day fun for someone.

Sometimes though, the spectator gets carried away and enters into the inner sanctum.  They do the unthinkable by engaging players (or the GM) in a conversation or even sit down at the table.  Now sometimes there is a lull in the game and you just need to take a break, and talking at that time is perfectly acceptable.  Being invited to sit at the table and perhaps run a NPC is another way to insinuate yourself into the fun without interrupting.  And there are just times when the interruption can't be helped due to time constraints, emergency situation or something that just needs to be done at that moment.   But as an overall rule, watch don't engage please.