The film was originally slated to be released in October 2007 for a Halloween release. Warner Bros., without explanation or reason, pulled the film from the schedule and no reschedule date was announced. One reason might have been that Saw IV (2007) was slated for release around the same time and Warner Bros. did not want to compete against it in fears of a big opening against them. Another possible reason the film was buried by the studio was that it was possible fallout from the box office disappointment of Superman Returns (2006), which was co-written by Trick 'r Treat writer/director Michael Dougherty and both produced and directed by Bryan Singer, whose production company produced this film. There was brief talk that the film would finally be released around Halloween 2008 and possibly early 2009 but neither release ever materialized. The film was finally released straight to DVD in October 2009.
The film had many title changes before filming began. Originally titled Season's Greetings (also the name of the short film), it was changed because it sounded too much like a Christmas film. Then it was called Halloween Terrors, Jack O' Lantern Tales, October the 31st, and Trick or Treat. The final title was kept but since there was already a movie of the same name out, they changed the spelling to Trick 'r Treat.
The name of the character Sam is derived from Samhain, the ancient Celtic festival of the dead from which modern-day Halloween is derived. Despite having been portrayed as such in this film and several other works, Samhain is not a deity or other personage of ancient Celtic polytheism; it is only the name of the celebration. However, the idea of being either a deity or other personage dates back to at least the late first century, largely thanks to an Irish bishop named Cormac mac Cuilennàin and his book "Sanas Cormaic" (Irish Gaelic for "Cormac's Narrative"), which had identified Samhain as such.
Movie reference: Me. Kreeg says "you've got to be fucking kidding me" when he sees part of the supernatural killer (its hand) operating independently from his body... This is a nod to a similar scene in John Carpenters remake of "The Thing" (1982).
The trivia items below may give away important plot points.
When the girls are buying their costumes, Danielle tells her date to meet them at "Sheep's Meadow." This foreshadows their later reveal as werewolves as it plays off the idiom, "A wolf in sheep's clothing."
In the scene when Laurie and her friends reveal that they are werewolves the background music plays Sweet Dreams by Marilyn Manson which is also played in a scene and in the ending credits of House On Haunted Hill (1999), later on Mr. Kreeg flips through a few channels and one of the channels is playing House On Haunted Hill (1959)
The chocolate bar that saves Mr. Kreeg's (Brian Cox) life in the climactic fight scene by stopping him from being stabbed by Sam is labeled as a "LHOTKA" bar, the last name of the Co-Producer, Peter Lhotka.
During the scene when Laurie's sister calls her while she sits alone in the street, in the background at the campfire, a remix of the song "Cry Little Sister" can be heard playing. That song was part of the soundtrack to the 1980's classic The Lost Boys. This was possibly used as both a nod to that movie as well as a foreshadowing of what was to become of the victims at the bonfire, as The Lost Boys contained a scene in which a group of young men were massacred by vampires at a bonfire similar to the one in this film.