One problem with serial killer movies is that the victims are over-motivated to stop the killers. A serial killer is so obviously evil that opposing one is a no-brainer. If fears are perfectly rational, then your story runs the risk of being too generic. If you want to write a story that’s specific to your character’s psychology, then make your character’s most irrational fears come true.
Every pregnant woman suffers the occasional fleeting fear that this process is actually entirely unnatural and there’s a monster growing in her belly… That’s why Rosemary’s Baby resonates so powerfully.
And this doesn’t just work for horror movies. Who hasn’t idly wondered, in their more paranoid moments, “What if everyone around me is in on a secret? What if my life is secretly being manipulated for the amusement of others?” The Truman Show made those fears come to life in an especially trippy way.These movies work not simply because the audience worries about Rosemary and Truman, but because we feel a creeping sense of dread that our own craziest fears are being exposed to all the world and made manifest. We can’t help but wonder: if this fear can come true, what’s to stop all the others??