We have all been sitting out at one stage when we seen a sketch that you know, but no one is guessing. In this situation you should sit in and WAIT for someone else to guess it, guessing yourself at the last second thus "saving" the sketcher. If you don't do this you are STEALING the sketch and this is very irritating to someone that was typing.
For those of you who have no noticed (new players) when you sit in you reset what someone has been typing, giving the person sitting in an advantage.

Play fair and wait for others to guess.