The last time I rode Haunted Mansion, there were only a couple people of other people, and my DH and I in the stretch room, huddled in the "dead center of the room". When the lights went out, the CM came up to me and said "BOO!" Yes, I screamed, but then laughed about it.
It does seem creepier when there isn't a crowd in there, doesn't it? Can you imagine what it would be like to be a single rider and end up in the stretch room alone?