In A Monster Calls a young boy named Conor (Lewis MacDougall) makes friends with a mysterious being that looks like a giant tree. That “monster,” played by Liam Neeson, then guides Conor as he copes with the illness of his mother (Rogue One’s Felicity Jones) and strict rule of his grandmother (Sigourney Weaver). Being a central figure in the film (his name is right there in the title!), he gets a lot of screen time. And being a walking bundle of roots, he had to be created almost entirely with practical effects and CGI. Some 45 minutes of A Monster Calls, out December 23, contain visual effects and a lot of them required Neeson giving a motion-capture performance. “I was involved with the motion capture for essentially two weeks,” Neeson says, “and most of my scenes were acting with Lewis.” Find out how the movie’s VFX team brought it all together in the video above.