The upcoming film's narration was derived from the book The Good Nurse: A True Story of Medicine, Madness and Murder, published in 2013 by Charles Graeber. Graeber interviewed Cullen, highlighting the shortcomings of the hospitals that housed an unsuspecting murderer for sixteen years.
The author also met Cullen’s colleague nurse, Amy Loughren, who was acknowledged for her life-risking courage in working with detectives to uncover the truth about the masked killer.
Graeber stated in his book that on November 2003, Loughren discovered that her colleague was constantly ordering a high number of drugs used in the cardiac ward while he worked in intensive care. The supply drawers often ran out, and Cullen would order restock from the pharmacy and given his position, he was often the first to sign the delivery.
Loughren’s suspicions only grew when two detectives contacted her about a sequence of patient deaths and unusually high levels of digoxin, a heart drug in their systems.