A monster is calling. YA author Patrick Ness’s story of a boy named Conor who is visited after midnight by an ancient monster stalks into theaters on December 23. The Focus Features film stars Lewis MacDougall as Conor; Felicity Jones as Conor’s mother; and Liam Neeson as the monster; J.A. Bayona takes the helm as the film’s director. Patrick Ness himself wrote the screenplay.

The novel, which Ness developed from an idea by the late author Siobhan Dowd, is illustrated by Jim Kay, was published by Walker Books in the U.K. and Candlewick Press in the U.S. in 2011. A movie tie-in edition and a special collector’s edition are also now both available. In the story, 13-year-old Conor O’Malley, who lives with his single mother in England, has been plagued by a recurring, unspeakable nightmare since his mother became sick with cancer. But one night, a real monster comes walking, taking the form of the yew tree that grows in the graveyard that Conor can see from his window.

The story behind Ness’s writing of A Monster Calls is itself a poignant one. Dowd, a British author and activist, conceived of the concept for the book while she was grappling with her own cancer diagnosis. She had planned to work with editor Denise Johnstone-Burt at Walker Books – who also worked with Ness – but Dowd was unable to see the project through. She died in 2007.

After her death, Denise Johnstone-Burt approached Ness to write the book that Dowd had planned, a proposal that Ness had initial trepidations about. In the introduction to the book, Ness speaks about this hesitation: “What I wouldn’t do – what I couldn’t do – was write a novel mimicking her voice. That would have been a disservice to her, to the reader, and most importantly to the story. I don’t think good writing can possibly work that way.” Ness’s introduction goes on to describe the change that happened as he began thinking about Dowd’s story concept: “Almost before I could help it, Siobhan’s ideas were suggesting new ones to me, and I began to feel that itch that every writer longs for: the itch to start getting words down, the itch to tell a story.”

Now that story is taking the leap from the page to the screen, with a screenplay written by Ness. He recently spoke with PW about A Monster Calls and what it was like to adapt his book into a film.

Could you talk a little about the process of adapting the novel to the screen?

I’ve adapted a few things now, and the one thing that keeps coming up is how very much movies aren’t novels. Seems obvious, but even in terms of length and impact, a movie is at most a long short story. So that's the challenge, really: how do you filter a novel to have the same impact in a shorter span? The upside, of course, is that pictures do so much storytelling, so it’s finding that right combo that works. A Monster Calls is a short novel and the structure works for the screen, but the challenge really is to keep it truthful to Conor and his emotional experiences.

That’s also the reason I wanted to write the screenplay myself (which I just went ahead and did; I always say writers should never ask permission, they should just write). I felt like I knew how the novel worked, and I worried that things might be changed and softened to make it work less well. I at least wanted to start the conversation about the movie and say, this is how I see it. And then have a filmmaker (which I am definitely not) respond. And we’d go from there. Fortunately, Bayona got the book immediately, and off we went!

Did you end up making any significant changes to the story as it transitioned into a screenplay?

There are compressions, certainly, but again, I was lucky that it’s a short novel! Nah, I’m kidding, there are changes, yeah, there have to be. The main aim, I feel, of the best movie adaptations is keeping the spirit of the book, keeping the emotions of it and all the things we love. For me, it’s akin to remixing a song!

And could you describe any particular challenges that came up or moments that might have been difficult to translate from page to screen?

You film everything, but then in the editing room, you realize what’s working best to always, always drive the story forward. Plus – and this is the great thing about collaborating – the filmmaker will bring their own ideas, too. Bayona had some tremendously moving ideas about legacy and what you leave behind for your kids. It meshed perfectly with the book and with the visuals he wanted. It’s what I’ve always believed about this particular novel, that it’s a baton I took from the wonderful Siobhan Dowd, then handed to our brilliant illustrator Jim Kay, and together we all made this book. That's how the film was. More baton-passing!

Did the illustrations in the book inform the visuals in the film?

Yes! Jim was a visual consultant on the film, and Bayona also talks about how he and Eugenio Caballero – our Oscar-winning art director – went on a journey to find the movie’s monster and eventually came back to Jim’s because it’s so strong. And just wait until you see how they’ve filmed the tales in the book. Eye-poppingly beautiful.

Can you comment on the overall experience of having A Monster Calls become a film?

I feel like I was really lucky on my first big film. The filmmakers were very collaborative, very interested in my input. I was on set over and over and over again. As a writer, it was a dream job. I learned a ton and felt included and appreciated. Plus, seeing my work onscreen is an amazing thing. I remember sitting on the side on the first day of shooting as the director did a walkthrough with Liam Neeson and Lewis Macdougall (who plays Conor) about the Parson’s Tale. I just sat there thinking, “I made that up!”