The film features two robots, CASE and TARS, as well as a dismantled third robot, KIPP. Nolan wanted to avoid making the robots anthropomorphic and chose a 1.5 m (4.9 ft) quadrilateral design. He said: "It has a very complicated design philosophy. It's based on mathematics. You've got four main blocks and they can be joined in three ways. So, you have three combinations you follow. But then within that, it subdivides into a further three joints. And all the places we see lines—those can subdivide further. So you can unfold a finger, essentially, but it's all proportional." Bill Irwin voiced and physically controlled both robots, with his image digitally removed, and Josh Stewart replaced his voicing for CASE. The human space habitats resemble O'Neill cylinders, a theoretical space habitat model proposed by physicist Gerard K. O'Neill in 1976.