Before Lance (Jack Black) jumps into the pool to save Shaun (Colin Hanks), he takes off his socks. This was not in the script, but was an idea of Jack Black's wardrobe assistant. Black found the idea hilarious, and the gag remained in the film.
Every time you see someone with a copy of Shaun's story, its just a copy of the movie script.
Jack Black is screenwriter Mike White's next-door neighbor. His part in the movie was written specifically for him. Surprisingly, little was improvised.
Despite the film title and subject, none of the film was actually shot in Orange County.