Optional rule I heard on D6 Generation podcast.
This may or may not be in the game but sounds cool to try out.
You can only talk to the other players if you are in the same square on the board. If you are not in the same square you will generate noise and that means zombies are coming your way. If you are one square away the person talking generates noise. The farther away you are from the player talking the more noise generated. So this is one way to keep people in a co-op game from telling everyone how to play the game.
Sign language and body gestures are, for the most part silent.