In addition to being an actor and screenwriter, How I Met Your Mother alum Jason Segel is also a successful children’s author. His first book series, Nightmares!, which he co-writes with Kirsten Miller, is about a young boy whose worst nightmares come to life. And the pair recently made the jump into YA, with Otherworld. To understand his attraction to the genre, we spoke with Segel after the premiere of his Sundance film, Come Sunday, about writing for kids.