In July 2006, less than six weeks before the start of shooting, the Henson-built monster suits arrived at the Melbourne soundstage where Spike Jonze and his crew had set up their offices. The actors climbed inside and began moving around. Right away, Jonze could see that the heads were absurdly heavy. Only one of the cast members appeared to be able to walk in a straight line. A few of them called out from within their costumes that they felt like they were going to tip over. Jonze and the production crew had no choice, but to tell the Henson people to tear apart the fifty-pound heads, and remove the remote-controlled mechanical eyeballs. This meant that all the facial expressions would have to be generated in post-production, using computers.
Spike Jonze turned down the chance to make the movie fully animated, because he wanted people to feel the Wild Things, and thought it would be more exciting and dangerous if a real kid were running around with the wild things.
The creatures are being portrayed by actors in six- to eight-foot tall costumes with some additional animatronics and computer-generated faces. The costumes were created by The Jim Henson Company, who is responsible for the Muppets and the creatures from Labyrinth - Dove tutto è possibile (1986) and The Dark Crystal.
Maurice Sendak personally favoured Spike Jonze as director, noting he was "young, interesting and had a spark that none of the others had". Jonze kept in close consultation with Sendak throughout the process, and the author approved creature designs created by Jim Henson's Creature Shop.
The movie's release generated conflicting views over whether it is harmful to expose children to frightening scenes. Spike Jonze indicated that his goal was "to make a movie about childhood" rather than to create a children's movie. Dan Fellman, Warner Brothers' head of movie distribution, noted that the film's promotion was not directed towards children, advising parents to exercise their own discretion. In an interview with Newsweek, Maurice Sendak stated that parents who deemed the film's content to be too disturbing for children should "go to hell. That's a question I will not tolerate" and he further noted "I saw the most horrendous movies that were unfit for a child's eyes. So what? I managed to survive."
Mark Ruffalo: In Max Records' previous film The Brothers Bloom (2008), he played Ruffalo's character at a young age.