Was Not Meant to Know. You must contribute to the mood. The more you roleplay your character's fear, the more that fear infects everyone else. Remember, everyone dies in a horror show except the virgin. Its just a question of who goes first.
Common themes are a dinner party, antique auction, book signing, art show, or even a holiday party.
Usually the guests don't realize something is amiss until well into the evening. The plots are subtle, and only the keenest observers will get a hint of what's coming. This is primarily a game of interaction. The characters will have to piece together clues to understand what is happening. Fear will lead to desperation, and then to panic as time runs out.