The newspaper clipping that Tom finds in her father's "important papers" bag was not created as a prop for this movie; it was a real news article titled "A Unit Stalked by Suicide, Trying to Save Itself" by Dave Phillips, which was one of the above-the-fold front-page articles in the New York Times on Sunday, September 20, 2015. The article was about a single Marine unit (the Second Battalion, Seventh Marine Regiment, aka the 2/7) that after returning from deployment in Afghanistan experienced a very high rate of suicide among its veterans. It is never explained in the movie whether the glimpse of this article is meant to imply that Will actually was a member of the 2/7, or if he just kept the article because it related to his PTSD and related medical situation.
The last third of the story was filmed at Squaw Mountain Ranch, a family oriented nudist resort outside Estacada, Oregon. It's a former logging camp, and the film needed a location with old cabins and RVs. Some of the members were extras in the "Birthday Party" scene. Established in 1933, Squaw Mountain Ranch is the oldest nudist club west of the Mississippi. It is open year-round, with rooms to rent in their lodge.
Once Ben Foster had signed onto the film, he and Debra Granik worked together to remove around 40% of the dialogue. This was to make the film have less exposition and feel more realistic.
The recurring theme of the seahorse is a callback to Will's disorder.
- PTSD is caused by a "blown-out" hippocampus, which in turn is caused by an over-stimulated amygdala, the organ that produces terror. In a healthy brain, the hippocampus double-checks the source of an alarm and switches off the amygdala if not judged urgent. This is nearly instantaneous, so that the person may not even be aware a terror prompt has occurred before the hippocampus cancels it.
- In PTSD sufferers, the hippocampus has been burned out by one or more intensely traumatic events, so that the amygdala keeps pumping terror prompts into the neural system over and over, with little or no interference. Hence the experience of flashbacks (sudden irrational terror originating from a trigger sensed by the amygdala, but possibly not even known to the victim).
- "Hippocampus" is also the classic/poetic word for seahorse. The hippocampus in the brain is called that because it's shaped like a seahorse.
In preparation for the film Ben Foster received training from a professional which included gaining wilderness appreciation, survival techniques, learning the basic fundamentals of water catchment and gray man technique which is how to disappear in public, or more importantly, how to disappear in plain sight.