You don’t want to casually watch this crossover indie horror success story like you would most things on your TV — your phone in hand, live tweeting and texting out cat photos. You probably won’t be able to. David Robert Mitchell’s fright-a-thon commands your attention not only because it’s good, but because it trains you how to watch it. As various teens try to avoid zombie-like creatures only they can see, you’re slowly conditioned to scan every widescreen frame, keeping an eye out for any rando who suddenly makes a slow beeline for our heroes. (Think of it as a freaky, cinematic version of “Where’s Waldo?”) Glance down even for a second and you’ll miss out. So don’t.