They can be chainsaw-wielding mainiacs, creatures from another dimension or supernatural presences. Horror film monsters frighten, haunt and shock us. They personify our biggest nightmares. And the actors that truly make those characters come to life on the big screen often give performances so convincing that—for just a minute—you forget they're only imaginary. Here are some of the actors and actresses who have given performances so real, they made viewers think twice about turning off the lights at night.