During the title sequence, several images from Crispin Glover's book Rat Catching are used. In one shot a copy of Rat Catching - a red colored, hard bound book - can actually be seen.
All of the photos in the house of Willard's father, Martin Stiles, are of actor Bruce Davison, who played the part of Willard in the original movie (Willard (1971)).
The movie prominently features the song "Ben" by Michael Jackson, this song was written specifically for the movie Ben (1972), which is the sequel to the original Willard (1971).
There have been reports that the enormous rat Ben was not portrayed by a rat, which is not quite true. Ben was played by a giant Gambian Pouched Rat (cricetomys gambianus), a distant relative of the Norway rat (rattus norvegicus), the species in the film. Gambian pouched rats, while fairly docile, can grow to body lengths of as much as 10-17 inches from head to base of tail, with a tail about the same length again or longer, and generally weigh anywhere from two to over six pounds.
The film was originally conceived as a R rated picture and was shot with that rating in mind. However, following the success of other PG-13 rated horror movies like The Ring (2002), executives at New Line asked the director to tone down the violence and language for the more commercial lower rating. This was done by shorting the demise of R. Lee Ermey's character Mr. Martin and cutting some of his lines.
The part of Cathryn was written for director Glen Morgan's wife Kristen Cloke. Cloke got pregnant and had to withdraw. She played a small role as a psychiatrist, though. That scene was cut during post-production, but can be seen on the DVD-release.
The film was written on the set of The One (2001) with Doug Hutchison in mind for the title role.
When Cathryn brings over her ginger cat to cheer Willard up, the cat is named Scully. Glen Morgan and James Wong served as long-time writers/producers on "The X Files" (1993).
The trailer contains some lines not used in the movie.
DVD contains various deleted/alternate scenes and an alternate ending.