Observers are still considered "players" in Volity parlance, even if they're not actively playing. They're still hanging out at tables, which is close enough for us.
If a game involves hidden information, then that game's observers get a different view of the game than anything the players see. Since observers are not playing, then they won't receive any information that's privy only to certain players; in a Poker game, observers would be able to see player actions (drawing, betting, and so on), and any face-up cards visible to the whole table, but not the cards in any player's hand.
To "look over the shoulder" of an active player, one just join that player's seat... and therefore, no longer be an observer.