The T-1000, also known as a Prototype Series 1000 Terminator, is a fictional shapeshifting android assassin, serving as the primary antagonist in Terminator 2: Judgment Day, and Terminator Genisys.
Created by the franchise's long-running antagonist, Skynet, the T-1000 is described in Terminator 2 as being composed of a mimetic polyalloy (nanorobotics), whose body is composed of liquid metal that allows it to assume the form of other objects (commonly knives and stabbing weapons) or people, typically terminated victims. Therefore, it is portrayed by multiple actors in the film. It is further explained in the prologue of the film's novelization, that the T-1000 was created through nanotechnology, and is a 'Nanomorph', able to scan the molecular structure of whatever it is touching and visually mimic it.
Billy Idol, and storyboards had the robot resembling him, but a serious motorcycle accident prevented Idol from accepting the role. Then, he thought of casting actor Michael Biehn, who played Kyle Reese in The Terminator, in the role with the explanation that Skynet managed to clone Reese's body and use it for a new Terminator. Cameron ultimately dropped this idea after deciding the audience would find it too confusing.
Eventually casting came down to Robert Patrick, as a deliberate contrast to the original Terminator: "I wanted to find someone who would be a good contrast to Arnold. If the 800 series is a kind of human Panzer tank, then the 1000 series had to be a Porsche." For a more machine-like performance, Patrick had to learn how to both fire a pistol without flinching (or blinking) and run effortlessly without heavy breathing or signs of exhaustion.