In his second book for young readers, The Sacrifice Box, Scottish writer Martin Stewart mixes humor and horror with coming of age. In the summer of 1982, five unlikely friends each leave a meaningful belonging inside a strange stone box to memorialize their time together, creating three simple rules: “Never come to the box alone. Never open it after dark. Never take back your sacrifice.” Four years later, the sacrificed items make a terrifying return, and the protagonists must grapple with their past. Stewart spoke with PW about his inspiration for the book, ’80s music and pop culture, and how he builds atmosphere and tension.

Your first book, Riverkeep, was based on a newspaper story about a family that cared for the Clyde River in Glasgow. What inspired The Sacrifice Box?

The idea for it had come initially just as a tiny seed. I had pitched it to my editor the first time I met her. At that point it was some kids who find this box in a forest and keep putting things in it, and it takes more and more and then starts spitting things out. All I had was the compulsiveness of this constant giving and it never being enough. When Riverkeep was finished and we were talking about what was coming next, she asked about the idea again.

I had the title, and my editor said that the problem was that it was a middle grade book—the original seed was this thing as a means of writing about childhood and change and how difficult that can be—but the book had to be YA. The YA is how the ’80s connection came about—the Breakfast Club reuniting to fight the Gremlins. It all fell together naturally; by coincidence those are both big ’80s films. The box would be something that these kids had done when they were 10 and 11, so I got to play with that Gothic idea of the past coming back to get you.

Riverkeep was lauded as a sort of macabre fantasy, whereas The Sacrifice Box employs clear horror tropes. If the two exist on a continuum, what was it like to move between them, and was there anything difficult about writing more full-fledged horror?

Yeah, the shift between them was interesting, because I didn’t plan for my first book to be set in a world of my own invention. The thing I had written was about scary clown puppets and things like that. When I got the deal for Riverkeep, the publisher bought a four-page story that I had written, and I had to keep it in this world. [For The Sacrifice Box] I had been looking forward to coming back to a world where I could reference a school bus, and make cultural references and things. So that was, in terms of moving from the fantastical to a more grounded version of that, a lot of fun. The thing I really wanted to ground it was the school experience. Everybody knows what it’s like to be bored in a classroom.

In terms of horror, I really like some Stephen King, but I started his son Joe Hill’s book Heart-Shaped Box and had to stop reading—it scared the bajeezus out of me. And I don’t watch a lot of horror films, so I wasn’t so much self-consciously exploring horror. In terms of what I’m aware of doing, it was just the language and pacing it correctly—that was a learning experience. The first draft of Riverkeep took six weeks, and the first draft of this one took a year and a half. Now that they’re written, I don’t feel that there is a huge difference between them—there’s a balance, and they feel like they come from the same place.

Atmosphere plays an important role in both of your books. How do you think about place and atmosphere, and how do you use it to contribute to a book’s other elements?

It is important to me. In the Lord of the Rings series, there can be pages about a hillside. This obsession with worldbuilding, I feel that I could do that quite easily, because I get into the sensory elements of it. One moment in the The Sacrifice Box I cut it down from six or seven pages. It’s a process of condensing; I tend to overwrite in an early draft. [Regarding setting,] in Riverkeep it was Glasgow, and the idea of the river—a quasi-Victorian industrial space. The map in the beginning is the boundary of the city of Glasgow, and I wanted to see if I could write something that would have a Scottish or Glaswegian energy that would be recognizable to people from here. In Scottish fiction, I think, there’s quite an interesting question of identity.

For The Sacrifice Box, the setting is an entirely fictionalized version of the Isle of Arran. My grandmother was taken there in the Second World War as an evacuee. A reason to cheat a little with the geography is I wanted the place to be British but to have a mid-Atlantic movie-land Americana to it. The way that we think of the ’80s is so heavily influenced by film, and there’s a kind of iconography that I wanted to steal from the movies—a British feel for American readers, and something that for British readers would have a feel of American movies. [In terms of atmosphere], what I am trying to do with The Sacrifice Box is find a fragile balance between really scary and a kind of beautiful humor. I’m not an instinctive horror person, but that kind of tightening of the screw in terms of a sensory experience and the pace of a thing comes from the execution of language. And in terms of trying to capture a space that would allow for that heightened sense, that’s why it’s so hot in the book—I wanted to take things to an extreme that would allow everything to happen against a high pitch.

The initial driver of writing the book was trying to capture a feeling. That was the creative lightning in a bottle—I wanted the story to build a shape that would have a funny friendship and an emotional payoff and still be about a homicidal teddy bear.

What would you put into the Sacrifice Box?

If I was younger, like the kids in the book, I think music, probably. It would be a mixed tape or a mixed mini disc (I got a minidisc player, and it was one of the most formative things that ever happened to me).

I think me now—I’m 35. I lost my mother suddenly last year, so I’m thinking about how you remember somebody. It would maybe be something having to do with my mum. Or my daughter is a year and a half, so something to keep her safe.

The Sacrifice Box by Martin Stewart. Viking, $17.99 Aug. 28 ISBN 978-0-425-28953-2