Miriam Traore heard about the movie “It” this year and thought the killer-clown adaptation looked interesting. So Ms. Traore, a 33-year-old project manager and Jazzercise instructor in San Francisco, did what she always does when a horror flick intrigues her: She skipped the theater and went on Wikipedia.

“I want to know what the story is. I don’t want the story told to me necessarily,” said Ms. Traore, who hasn’t seen a horror movie in ages but recently read the plots to “Amityville Horror,” “The Boy,” “Halloween 3,” “Split”...