Joel McHale spent months training with knives and martial arts in order to do his own knife stunts in the film. He managed to go through months of training, as well as most of filming without a single injury. However during one of the last days of filming, his son was visiting on set and McHale used his knife to cut a piece of white chocolate for him. He cut his hand and bled profusely.
In the scene where Sarchie and Mendoza are sitting in the car waiting for Santino to come back to the apartment, on the wall behind the car it reads January 4th. January 4th 1967 is the release date of The Doors first album. The entire movie is filled with references to The Doors.
The Latin writing on the wall, "Invocamus te vi ingrediaris ab inferis" means "We call upon you with force to enter from hell". The sentence can also be seen for a few frames in the static at the end of the opening scene.
According to Scott Derrickson, Sean Harris went into a trance state when filming the forehead stigmata scene. After filming of that scene finished, Harris had his prosthetics removed in the make-up trailer. Derrickson, who was still on the set and about to leave, was approached by a concerned crew member, who told him that something wasn't right with Harris. When the director arrived in the trailer, he saw all make-up artists visibly scared and at a distance from Harris, who was speaking in a strange language. Once the remaining prosthetics were removed, he was driven home by crew members and brought to bed, and they stayed there until he fell asleep. The next day, as Harris arrived on the set, Derrickson asked him about what happened the other night. Harris told him that he had a "kind of expansion and then a pop" in his consciousness while filming that scene, and that he had no recollection of what happened during the hours afterwards.