It was crucial to the filmmakers involved with The Sixth Sense that the fact that Dr. Malcolm Crowe was actually one of the "dead people" be more than a tacked on surprise ending. They wanted to make sure that every scene would accurately setup the payoff ending. In order to accomplish this they created some detailed "rules" that would be strictly observed when filming each scene. Here are some of them:
The room only gets cold when a ghost is angry or upset.
Nobody ever talks to Malcolm.
With enough effort the dead people that Cole sees can affect their surroundings.
Malcolm only wears clothes that he interacted with the night of his death.
Both of the boys (Cole and the boy that shot Malcolm) have a lock of white hair.