While investigating a series of murders taking place both day and night in Burlington, Vermont, the BAU comes to believe it is the work of two serial killers with drastically different M.O.s who are operating simultaneously. Also, the team worries about Reid's safety in prison.
- Eddie Alfano - Frazier
- Diego Amarosa - Duerson
- Jhey Castles - Newscaster
- Drew Fonteiro - Jon Kreutzmann
- Phillip Garcia - Luis Delgado
- Jeananne Goossen - Fiona Duncan
- Alexander Harris - Officer Peters
- Gary Kasper - Samuel Gordon
- Ian Kerch - Paul McConnell
- Jeffrey Markle - M.E. Harry Greenberg
- Harold Perrineau - Calvin Shaw
- Matt Raimo - Mike Hood
- Emmanuelle Roumain - Janis Weir
- Heidi Sulzman - Sergeant Suzanne Kuroda
- Atticus Todd - Chess Inmate
- Johnny Wactor - Trey Gordon
- Ebboney J. Wilson - Lyn Anastasio
- Oleg Zatsepin - Bob Joplin
- Lonnie David Franklin, The Grim Sleeper (possibly; unidentified)
- Louis Craine
- Michael Hughes
- Daniel Lee Siebert
- Chester Turner
- Ivan Hill
- The Southside Slayer
- Mark Goudeau, The Baseline Killer
- Dale Hausner and Samuel Dieteman, The Serial Shooters
- The Russian Mafia
- Jennifer Jareau: "Life, of course, never gets anyone's entire attention. Death always remains interesting, holds us, draws us." – Janet Malcolm
- Tara Lewis: "How blessed are some people whose lives have no fears, no dreads, to whom sleep is a blessing that comes nightly and brings nothing but sweet dreams." – Bram Stoker
- "In the dark" is an idiom that refers to being in a state of ignorance about something.
- This is the sixth of nine episodes to deal with two separate cases.
- Gary Kasper, the actor portraying Samuel Gordon, previously appeared in the Season Six episode "Supply and Demand", in which he portrayed The Cleaner.
- A number of characters featured in the episode were named after members of the rock bands Phish (whose birthplace was also Burlington, Vermont) and Grateful Dead.
- Stephen mentions instances of serial killers active at the same time in Los Angeles, but he does not elaborate any further, with Franklin being the most likely candidate. This applies to the below six killers.