Visual effects have come so far, so fast that just about anything a director can imagine can be conjured up by computers and projected on the big screen. The new problem for filmmakers doesn’t have to do with constraints, it has to do with the fact that most of the really cool, seemingly impossible stuff has already been done.

Such was the challenge facing director J.A. Bayona when he began work on adapting Patrick Ness’s dark fantasy novel, A Monster Calls. The film’s young lead, Conor (Lewis MacDougall), spends much of his time talking with a gigantic, old tree (voiced by Liam Neeson) that has somehow come to life and decided to grant him three wishes. In the last decade, sentient trees have been highlights in multiple mega-sized hits, including the Lord of the Rings series and Guardians of the Galaxy. Bayona had to find a new way to stick out from the crowd.

“You go on the internet, and everything about tree monsters has been done already,” Bayona told Inverse after his film premiered at the Toronto International Film Festival. “It was very difficult, after doing 200 designs, to the book’s illustrations by Jim Kay.”

But knowing what the tree would look like was just the first step. Bayona had to figure out how to make it come alive. After some debate, the Spanish filmmaker — who will next direct the Jurassic World sequel — decided to mix the new tech with some advanced versions of old-school movie magic: Animatronics.

“Because it’s a movie that mixes very different tones, we had to keep the fantasy very grounded,” he said. “The ultimate meaning of the story is how we need fantasy to cope with reality. … I didn’t want a lot of CGI to distract from the more dramatic stuff.”

With the tree stomping around the small British town where the movie takes place, and looking through Conor’s window — which was the size of the tree’s eye, by the way — the scale of the model was a big challenge.

“The animatronic was huge, life-sized,” Bayona said. “We had a big head that was able to move and talk, and had movement in his eyes.”

They wound up using plenty of CGI too, of course — no film today goes without some kind of computer visual effects work — but made sure to fully match the textures of the animatronic, so they could seamlessly switch between the two in post-production.

There is another, wholly different element of animation in the movie as well. Painterly images capture Conor and his tree in more impressionistic tones inspired by the illustrations in the original book. Those two mixed different types of visual effects.

“The animation, as it moves forward, wants to be more real and real, telling the audience that the fantasy is becoming more real to Conor than reality,” he said. “We started with 2D animations that turn out to be 3D animations, and there are moments where the animated stories are taking part in reality.”

A Monster Calls, which also stars Felicity Jones and Sigourney Weaver, hits theaters on December 23rd.