(at around 9 mins) The kid with the lollipop in his mouth, who peeks in on the girls in the changing rooms when they are trying on Halloween costumes is Quinn Lord who plays Sam throughout the rest of the movie.
The performers under the masks playing the kids on the school bus all have real disabilities or injuries, and they apparently had a blast during the shoot. Their costumes were inspired by a mix of photos from Ralph Meatyard and Diane Arbus capturing real mentally disabled children on Halloween in their own homemade costumes.
Most of the Jack O' Lanterns were made out of either foam or ceramic. An on-set joke was that no pumpkins were harmed during the making of this movie.
They couldn't use any recognizable candy wrappers- "because all of our candy in this film is either poisoned or filled with razor blades" and had to create their own instead "which was kind of fun because we came up with stuff like Big Fudge Log." Michael Dougherty revealed.
There were 18 drafts of the script, and one theme that crept in along the way is the idea that each of the stories represents a different stage of a person's experience with Halloween. "The first story with Dylan Baker and his son is about how you're introduced to the holiday. The second one with the kids is you and your friends roaming around without parents for the first time. This one with Anna [Paquin] and her friends is Halloween in your 20s when it's about sex. The final story with Brian Cox is Halloween in your twilight years. It's the Scrooge of Halloween."