Brando's arrival on set was severely delayed for personal reasons (Brando's daughter had suddenly committed suicide), and Brando had been one of Stanley's most vocal (and only) supporters. So Stanley was forced to try to film around Brando's absence, which meant trying to get all of Kilmer's scenes done. This proved to be difficult, because Kilmer, who was an actual movie star back in 1995 and not the confusing punchline he is today, was fighting the director at every turn, issuing an endless list of movie-star demands. One such demand was the construction of a treehouse, which Kilmer felt was the only natural place his character would want to stay. Stanley pointed out that there was no treehouse in the script, and that they were not about to build a treehouse just to accommodate him. Kilmer responded by making sure Stanley got fired from the film.
Stanley, however, did not go quietly. He trashed all his notes, storyboards, and production art so that whoever was brought on as his replacement would not be able to use any of his ideas. New Line had him escorted to the airport to fly him far away from the production before he caused any more damage, to which Stanley heroically responded by not getting on the plane and instead scurrying off into the Australian rainforest, where he hid for several months. (Actual quote: "I lived for two months underneath a tree with two dingo pups, figuring out what the heck to do.")
Eventually, a few members of the production still loyal to Stanley found him in his jungle retreat and took him back to set disguised as one of the film's many animal extras. You can actually see him in the finished film, shambling around in a giant dog mask mere feet from Val "I want my treehouse" Kilmer. Only a few people had any idea Stanley was there; the rest, including Kilmer, Brando, and the studio heads, thought he had simply disappeared into the ether. And so, beneath a rubber dog mask, Richard Stanley got a front row seat to watch the movie he got fired from become one of the most famous disasters in cinema history.
The mask looks like how watching the movie makes your brain feel.