I would like to fourth(third? fifth) the point that the character's reactions are what make a thing scariest. It's one thing to see what they see, but If we're sufficiently in the head of the MC, I'm going to be feeling what they're feeling too. I was not a child the first time I read IT by Stephen King, and honestly a straight up description of the various forms of ol' Bob Gray wouldn't have bothered me, being a product of horror books and movies galore, BUT the way he brings us into the characters minds breaking apart as they take him in scared the crap out of me.