In Stephen King's novel, Judd mentions that a dog went wild in a nearby town and killed several people. This is a reference to the events of Cujo (1983), another novel by King. It is common for characters in King's novels to mention the events of his other novels.
The role of Zelda, Rachael's dying sister, was played by a man. Director Mary Lambert wanted Zelda and her scenes to frighten the audience but did not believe that a 13-year old girl was scary so she cast Andrew Hubatsek in the role to make something be "off about Zelda".
The original screenplay featured the 'wendigo' (a Native American demon) that was mentioned in the novel, but it was ultimately cut from the film. Its presence is implied only twice: firstly, in the scene where Louis is walking through the woods at night and hears something large knock down a tree. Secondly, being when Jud first takes Louis up to the Indian burial ground. There is some kind of loud crash deep in the forest followed by a long, almost feminine howl. Jud says that "it's only a loon" but it's clear that he doesn't entirely believe it himself.
Louis claims his cat Church is named after former-British Prime Minister Winston Churchill. 7 blue British Shorthair cats were acquired to play Church, and each of them were trained to do a specific action for the camera.
The story was inspired by actual events experienced by Stephen King that occurred while he was living in Orington, Maine with his family. King recalled that while living there his daughter's cat was killed in the highway. Much of Ellie Creed's emotional outburst was taken directly from King's own grief-stricken daughter. King also remembered that once his youngest son had nearly run into the road while a truck was speeding down it, much like Gage does in film. The character of Judd Crandall was based on the elderly neighbor that lived across the road from King. Also there was an actual pet cemetery in the woods behind the King house, which became the basis for the one in the novel.
Stephen King is a big fan of the Ramones and referenced some of their songs in the novel Pet Sematary. In homage, The Ramones wrote and performed the theme song "Pet Sematary", which is featured in the films closing credits.
The trivia items below may give away important plot points.
The picture at Rachel's parents' house is a painting of Zelda as a child, before her spinal meningitis. Gage is later seen wearing a similar outfit (as well as having her red hair) to signify that Zelda has come back through him, which was Rachel's deepest fear.
The character of Missy is actually the blending of two characters into one. In the book Missy does not commit suicide. The only characters to die prior to Gage's death are Pascow and Norma Crandall, Jud's wife.
Judd Crandall's house for the film was actually a facade built upon a smaller preexisting house. For the finale, where the house is burned, an asbestos shield was constructed between the two houses so that while burning the facade no damage would occur to the smaller house it was built upon.