“It,” Stephen King’s sprawling novel about a band of friends confronted with a child-eating clown, is one of the horror master’s most terrifying works. Director Andres Muschietti, who first turned heads with 2013’s “Mama,” has been tasked with turning the doorstop of a book into two films. The first part hits theaters Sept. 8. Muschietti took a break from working on the picture to talk about directing a cast of teenagers, remaining true to the spirit of King’s novel and why he’s excited that this version of “It” can show more gore than the 1990 ABC television movie.

Was it hard to adapt such a massive book?

You have to start simple. You track the big emotional tentpole events of the movie. We tried to fill it with as much character and story as we could in a two-hour movie.

There will be two films?

We are doing that. We’ll probably have a script for the second part in January. Ideally, we would start prep in March. Part one is only about the kids. Part two is about these characters 30 years later as adults, with flashbacks to 1989 when they were kids.

What kind of response do you get from fans of the book when they find out you’re directing a film of “It”?

Most of the people are excited about seeing a good adaptation. There are naysayers. Those tend to be the people who are fans of the miniseries rather than the fans of the book. People who read the book and got the book, they’re not crazy about the miniseries. It was a very watered-down version. It didn’t contain the darkness that the book had. They couldn’t make something for TV about a clown who eats children.

“There are naysayers. Those tend to be the people who are fans of the miniseries rather than the fans of the book.” Andres Muschietti

This is rated R?

I’m so excited that it’s rated R. I don’t feel that we held back in any aspect.

Why did you pick Bill Skarsgard to play Pennywise the clown?

I wanted to stay true to the essence of the character. I knew that I didn’t want to go the road of Tim Curry [who played Pennywise in the TV miniseries]. Bill Skarsgard caught my attention. The character has a childish and sweet demeanor, but there’s something very off about him. Bill has that balance in him. He can be sweet and cute, but he can be pretty disturbing.

Did he stay in character in between takes?

He didn’t stay in character when the camera stopped, but we did try to maintain distance between him and the kids. We wanted to carry the impact of the encounters to when the cameras were rolling. The first scene where Bill interacted with the children, it was fun to see how the plan worked. The kids were really, really creeped out by Bill. He’s pretty intimidating because he’s six-four and has all this makeup.

How did you approach directing the child actors?

We made them bond way before shooting. We had them for 10 days doing activities together. There was technical stuff. We had bicycle camp because kids today don’t necessarily know how to ride bikes. Magically they all became friends.

Did Stephen King like the film?

He tweeted about it and said the movie exceeded his expectations. After that we started a private email exchange because I was so excited about his response. My first letter was me asking for indulgence and forgiveness for having changed things. The story is the same, but there are changes in the things the kids are scared of. In the book they’re children in the ’50s, so the incarnations of the monsters are mainly from movies, so it’s Wolf Man, the Mummy, Frankenstein, Dracula. I had a different approach. I wanted to bring out deeper fears, based not only on movie monsters but on childhood traumas.

What’s the key to a successful horror film?

Stay true to what scares you. If you don’t respect that, you can’t scare anyone.