The seance scene, especially the shots in the ghost's point of view as the he closes in on the group of five seated around the table.
While listening to the audio tape recording made of a spirit-writer's attempt to contact the ghost, the murdered boy's voice can be heard desperately whispering... something that was completely inaudible during the scene in which the tape was made.
The entire premise of the film. On the one hand, there is John who is agonising over what he could have done differently to save his daughter's life. On the other hand, there are people like Joseph's father who murder their own children for profit.
Captain DeWitt, the police officer working for Carmichael, adds a significant level of very real fear to the film in his one scene. Imagine you're trying to uncover something horrific solely because it's the right thing to do, with no ulterior motive aside from putting a ghost's soul to rest. Then imagine the person you need to communicate what you've found to in order to do that assuming that you're trying to blackmail him for money, so he sends a police officer to threaten you with being put in a mental asylum and declared insane unless you stop your investigation. DeWitt's sudden, shocking death after he leaves is just the icing on the cake.