Spike Jonze was approached by Maurice Sendak and was asked to take on the film adaptation. A movie had been on the burner since the early 90s, and Mr. Sendak had not been able to find anyone fitting to take it on.
The creatures are being portrayed by actors in 6-8 foot tall costumes, with some additional animatronics, and computer-generated faces. The costumes were created by The Jim Henson Company, who were responsible for the Muppets and the Labyrinth creatures.
Throughout the writing process, Spike Jonze and Dave Eggers kept in close contact with Maurice Sendak, consulting and sharing script changes, models of the monsters, etc.
It has been said that actors in the monster costumes would wear the head for no more than half an hour at a time, after which they would have 10-15 minute breaks in front of an air conditioner. Stunt performers would remain fully suited for hours at a time, without air conditioning.
Numerous cameras followed the actors around to impress their expressions and feelings. The images would then be digitally "infused" onto the faces of the puppets and would follow the actors' performances.
Michelle Williams was at one point cast to voice KW, but the role went to Lauren Ambrose. The reason given by the filmmakers for this was that Williams' voice didn't match what they're going for. Chloƫ Sevigny was another consideration for the same role.
Initially, Warner Brothers studio was so unhappy with Spike Jonze's final movie (it was much less family friendly than they imagined) that they wanted to re-shoot the whole $75 milion project in early 2008. Jonze was eventually given some more time and money by the studio in order to make the final product satisfying to both, the studio and himself.
Though their names are not mentioned in the book, Maurice Sendak named the Wild Things after his aunts and uncles: Bernard, Tzippeh, Aaron, Moishe, etc. In the film they have totally different names.
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 actors appeared 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 50-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.
All the original songs in the movie were written and preformed by Karen O, credited as her stage name Karen O., the lead singer of the Yeah Yeah Yeahs. She dated director Spike Jonze at the time of production. They have since broken up.
An early test scene made by Disney back when the movie was expected to be done through traditional hand-drawn animation, featured Max writing his name on the wall of his room with a marker and chasing his dog down the stairs. The opening scene of the 2009 film is similar to this, as it features Max chasing and wrestling with his dog. Though ultimately, the scene is an homage to the second picture in the book featuring Max in his wolf suit, chasing his dog down the stairs with a fork in his hand. Disney's scene can now be found on YouTube.
Spike Jonze had his voice cast act out their characters on a soundstage, to ensure that the voices came from a performance of actors working together, instead of filming the movie and recording the voices later, or recording the actors individually. The actors wore radio microphones attached to their heads with headbands. Jonze and Catherine Keener would perform Max. Even before this soundstage shoot, Spike took the voice cast to Griffith Park to wage an actual dirt clod war.
The early test done at Disney in the 1980s was a combination of hand-drawn animated characters and computer animated backgrounds. The test was done by Glen Keane and John Lasseter.
Sam Longley, the suit performer for Ira, is a Perth Western Australia actor famed for his comic and improvisational performance skills. He is also 6 foot 10 inches tall and brother of Luc Longley of Chicago Bulls fame.
When Max's mother is shown speaking on the phone to her co-worker, she asks why "Mr. Lasseter" (presumably her boss) didn't like her report. This is most likely a reference to John Lasseter, who was one of the original artists on the now famous "Where the Wild Things Are" Disney animation test.