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.
While it's widely assumed that Suki Waterhouse's character's amputated limbs were achieved digitally, most every shot is an in-camera practical make-up effect. Lots of advance planning and storyboarding was required, in order to pull off all of the shots required in a timely fashion.
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.
No soundstage work here. The entire film was filmed in extreme locations, and primarily outdoors in high heat.