May be it’s because of the scene shown on the cover, but I find the floppy sak mask in The Strangers absolutely terrifying and in this particular scene some people don’t notice the killer in the back for a while and while they’re waiting for him to attack or something… BAM! They realize the killer has been there the whole time. I think it’s the mask mixed with the personality of the person underneath it, like in this movie he’s so calm and plays with the girl’s mind, toying with her, driving her insane with paranoia, acting like he’s going to kill her, then never doing it until the end.