Horror writer Paul Tremblay ’93 won the Massachusetts Book Award in 2016 for his fifth novel, “A Head Full of Ghosts” — a book so frightening that Stephen King tweeted that it scared even him. In this episode, we talk to the author and AP calculus teacher at St. Sebastian’s School in Needham, Mass., about how he went from a master’s degree in math to writing fiction, his analytical approach to the writing process, and his advice for younger authors.