No soundstage work here. The entire film was filmed in extreme locations, and primarily outdoors in high heat.
Most of the extras hired were from Slab City, the same California community Chris McCandless visits in Into The Wild (2007). Most of the "Slabbers" came as is, not needing costumes or make-up.
Director Ana Lily Amirpour is a huge advocate for practical effects, which kept Make-up Effects Designer and Supervisor Tony Gardner on his toes throughout the entire production.
While filming this movie in Niland, California, Keanu Reeves stayed in Brawley, California. He was frequently spotted at various locations in Brawley, including Starbucks, Von's, and Snap Fitness taking pictures with the locals, and reenacting some of his movie roles.
The yellow smiley face shorts worn by Arlen are the same as Die Antwoord rap artist and 'Chappie" actress Yo-Landi Visser wore in the video for the song 'Baby's On Fire'. Die Antwoord's song 'Fish Paste' is also featured in this film.
Much of the filming was done in Bombay Beach, California. Which is "technically" still part of Niland, California, and many locals were cast as extras in the film.