A great little video exposé about what makes a horror film work well. The example used in the video is both the film Lights Out and the short film in which the movie was based (also called Lights Out).
When I first saw the short back in 2013, I was scared. Like, genuinely scared where I really didn't want to turn off any lights. The idea for the "monster" only appearing when the lights were off seemed new at the time. Sure, monsters always appeared in the dark, but putting that slight control in the hands of the person being scared really felt new and horrifying. It was that curiosity at play which also ended up being the viewer's demise.
If you missed the short, you can watch it below. I hear the movie is scary, but no way am I going to see an HOUR'S WORTH OF THIS.