Wrapping up two big projects is how YA author Maureen Johnson will end this decade and start the next one. The film version of Let It Snow, which she co-wrote with John Green and Lauren Myracle, debuts on November 8 on Netflix, and her Truly Devious trilogy, about a teen detective investigating a century-old murder at her Vermont boarding school, concludes with The Hand on the Wall in January. We spoke with Johnson just hours after her return from Los Angeles, where she and her co-authors attended the Let It Snow premiere.

Was this your first time seeing the film?

We all saw it two to three weeks ago, also in Los Angeles, because Netflix wanted us to be able to talk about it and they wanted to make sure we liked it, which we all did. We genuinely love it.

The novel came out more than a decade ago. Was there a specific holdup?

It was optioned by someone initially, I don’t remember who. Then it was optioned by Universal and the producer who optioned it for them wound up moving to Netflix, so it was a long process. But as soon as it went over to Netflix, things happened very quickly.

On Twitter you said that there was some excellent swag for those who attended the premiere: holiday-themed skull caps with pom-poms. Did you snag one of those for yourself?

Absolutely. Everyone who saw the movie got a hat. That wasn’t the most impressive thing, though. There was a party after the premiere at a diner and the carpet in the entrance said “Let It Snow.” I have no idea who puts these things together.

One measure of success will be reviews. Has the film gotten any yet?

Not to my knowledge. I assume it will turn up in a Google alert. Google alerts and Twitter are how I learn everything.

What’s your review, and your co-authors? Is the film faithful to the novel?

It’s significantly different than the book but they’ve improved it. In fact, it doesn’t bear much resemblance to the book at all. It’s a movie for 2019. But we all—me, John, Lauren—feel 100% behind it. They did a great job.

Turning now to 2020, you’ll start next year with the release of the conclusion to your Truly Devious trilogy, which your fans have to be very excited about.

It’s done!

It’s commendable how complex this story is and how you managed to churn out three books in two years. Did you have the whole arc of the story outlined before you began?

Yes, absolutely everything about this was planned. With mysteries you have to do that. This series started from the why. “Why?” is the question that drives the whole machine. But I had charts and graphs and I drew a lot of maps. I wrote an entire file of answers and explanations before I started writing. It’s against the rule of mysteries to try to write one as a series, so I had to have a delivery in each of the first two books. In the third book absolutely everything is resolved.

But I will never do this again. All my future mysteries will be contained in one book. That’s the correct way to do it.

You had written another trilogy—Shades of London—but it wasn’t a mystery. Was it easier?

It was different. It started from a different place, which was, ‘What if there were better quality ghosts?’ I had taken some historical tours of London and everything was haunted. I would ask, “How do you know it’s haunted?” and the answers were “Oh, there’s a cold spot in the room,” or “The door opens on its own.” These were really lame ghostings. I tried to imagine terrifying ghosts. Ghosts who mattered.

So no more mystery series but perhaps more mysteries?

I don’t know why I had never written a mystery before because my passion really is mysteries. I am a lifelong mystery buff. Maybe there’s an aversion to doing the thing that’s closest to us, that means the most.

Who is your favorite detective?

Well I certainly love Sherlock Holmes. The Hound of the Baskervilles has a very special place in my heart. I was a big Agatha Christie reader as a kid. Before I started Truly Devious, I made a list not only of all my favorite detectives—who would be [main character] Stevie’s favorites, too—but a chart of all possible motives for a crime.

Did you ever want to be a real detective?

I’ve gotten that question more than once in my life. As a child, I wanted to be Encyclopedia Brown although I realize now he really wasn’t much of a detective because it was always Bugs Meany. “How did you know that Bugs did it?” My answer was, “It was always Bugs!” I mean his name is Bugs Meany. It’s sort of a giveaway that he’s going to be the villain.

You’re very vocal on Twitter about current events. Do you plan to be active in politics next year?

Oh, yes. More than ever! This is going to be quite a year. In fact, I’m deciding what to do right now because now is the time to sign up. I’ve been having conversations about what I can do to get teens and young adults involved. I’ve been doing that since 2008, but now it feels a little more urgent. So yes is the answer, a hard yes but I’m not sure yet exactly what it’s going to look like at this point.

Good, then, to have cleared the decks with the film coming out and the series concluding!

Well, there will always be something due. I’m pitching a new book and I have some other books to finish but, yes, I have just the Truly Devious book coming out next year. I’m very lucky. I would never use the word blessed lightly but I feel that way. To be able to write all the time is all I ever wanted to do. Now my husband is kind of eyeballing me a little bit because he thinks, occasionally, it’s okay to put your computer down. There are other things to do. He’s English. He’s trying to teach me to relax, but I’m trying to help him understand that I can relax with my computer on.

The Hand on the Wall (Truly Devious #3) by Maureen Johnson. HarperCollins/Tegen, $18.99 Jan. 21, 2020 ISBN 978-0-06-233811-2