Adam's jar held a millipede, not a centipede. Bill Paxton asked Matthew McConaughey what a good name would be and he answered "Curtis". Thus the millipede was 'Curtis'.
Jeremy Sumpter, who was cast as younger brother Adam, originally auditioned for the role of Fenton.
Loosely based on the case of American serial killer Joseph Kallinger who murdered three people and tortured four families. He committed these crimes with his 13-year-old son Michael between 1974-1975 in New Jersey. Kallinger pleaded insanity, claiming God had told him to kill.
When Fenton is digging the dungeon (from 50:18 to 51:37) his T-shirt is wrapped around his head. At 50:15 in the director commentary Bill Paxton says "These [scenes] were designed in the storyboards. It was described that he'd put his T-shirt on his head by Brent Hanley." At 50:50 in the writer commentary Brent Hanley says "I like the shirt over the head. That's actually really cool. I think 'Matt OLeary did that himself." Since David Ivie drew the storyboards, the T-shirt on the head might even have been David's idea.
Bill Paxton's directorial debut. Allegedly, the title Frailty (which is never spoken in the film) refers not to the slow disintegration of the family, but to the very low budget for the movie. Lion's Gate considered changing the title to "God's Hands" (which is a phrase spoken in the film) but Paxton and writer Brent Hanley insisted against this. Ironically, the Brazilian title is 'A Mão do Diabo' (The Devil's Hand').
The demon-destroying axe is inscribed with the letters OTIS. The significance of this writing is never given. Fans have speculated that Otis Meiks is the true name of Bill Paxton's character "Dad." Others believe OTIS to be an acronym for Only The Innocent Survive.
Bill Paxton used the same beer can throughout the entire filming of the movie because they could only find one Hamm's beer can like that from 1979. The can was "opened" off-camera and a sound effect added for popping its top (director commentary at 52:21).
Bill Paxton mentions in the director commentary at 54:19 "We couldn't really dig it [the hole] that deep .. unless it's reinforced, so actually in this shot [54:31] I'm on my knees and kind of making it look like I'm standing up to make the hole look deeper and of course there's a little mound there [at the lip of the hole]."
At the title credit (01:12) in the writer commentary, Brent Hanley says "Frailty to me was always about the frailty of perception, the frailty of morality, the frailty of right and wrong." At 02:16 he adds "I liked the idea of an abstract title."
Putting the name OTIS on the ax was completely director Bill Paxton's invention according to Brent Hanley at 56:19 in the writer commentary. At 01:30:00 in the production commentary, it is noted that Paxton put the name OTIS on the ax to mark it so that the audience would know that it is an heirloom passed from father to son.
The scenes of the lengthy car ride of Agent Doyle and his witness, were shot on a sound stage. Director of photography Bill Butler had crew members slighting shaking the car and moving lights and props to create the illusion of moving through the night.
Bill Paxton insisted on flying the young Tennessee actor Blake King to the shoot in Los Angeles in order to have a boy with a native southern accent play the small role of Fenton's classmate Eric who invites Fenton to "wrap" Corey's house (at 40:55 in the director commentary).