When Beckett tries to convince Castle to go back to her place to take more "pictures" in bed, As Castle removes himself from temptation he turns and walks into the metal pole.. that was unscripted Nathan (Castle) actually walked into the pole, the reaction on Stana's (Beckett) face was genuine.
First item on Castle's bucket list is to "Be with Kate" and the list was made 3 years earlier.That was about the time she was with Tom Demming.
Sean Whalen (Leopold Malloy) made his film debut in Wes Craven's The People Under the Stairs (1991). Wes Craven makes a cameo appearance here.
At the first crime scene, Castle finds it creepy that, with the look of horror on her face, Val Butler "looks like an Edvard Munch painting". He is referring to Munch's "The Scream", one of the most famous paintings in the world.
"Death is only the beginning" is a classic horror quote taken from Mary Shelley's "Frankenstein".