The movie made many significant changes from the novel, particularly in the make-up of the family, which in the novel consisted of two parents and three children, as well as the dog, who sees the family as his pack, which must be defended at all costs. The dog's perceptions of events are treated in great detail, as is the relationship between him and his human family, and his confusion as to whether the werewolf is a threat to his family that must be eliminated, or a pack member who must be respected. These subtleties mainly did not make it into the film.
Ted's Airstream trailer is meant to suggest a silver bullet. "That's a nuance nobody ever notices." Eric Red stated.
The film's animal trainers play the animal control officers who arrive to secure Thor and take him to the pound.
The commentary track is available on the director's cut only, and Red explains the difference between this version and the theatrical. It came down to one scene that he cut for this preferred version, "a transformation scene where poor visual effects ruined a sequence where Ted turns into a werewolf." He's hated the scene for over two decades, and he says fans have been complaining about it to him almost as long.
The book is told through the POV of the dog, Thor, and while the film drops that approach "the dog who played the role had to be a great movie dog." Several dogs were used in the film, each with various skills, but "the hero dog," the one used for close-ups and general scenes took months to find. After seeing multiple dogs in person Red was sent a photo of an untrained German Shepherd and knew instantly that was his Thor.