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.
No soundstage work here. The entire film was filmed in extreme locations, and primarily outdoors in high heat.
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.
Miami Man's Bad Batch tattoo can be partially seen during a conversation in the desert. It clearly shows only two digits after the BB meaning he's one of the first 99 to be exiled.
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.
When Arlen is asking Miami Man where he's from she asks if Cuba is in the tropics, near Hawaii. Jason Momoa who plays Miami Man is from Hawaii.