Even when the script doesn’t add up, David Robert Mitchell’s second feature (after 2010’s The Myth of the American Sleepover) is an efficient fear-generating mechanism. Most horror films tap into a variety of fears, and It Follows is no exception. Mitchell’s scare tactics include water (it begins and ends in pools), sex (a virus that can be transmitted only by intercourse), and suburbs (evil lurks behind the placid facade). But the primary fear is that of being followed—someone is walking behind you, so you start walking faster, change course, or cross the street. Chances are they’re just minding their own business, but what if they aren’t?…