That concept came directly from the thinking Schwarzenegger had described at lunch. The actor who initially wanted the human hero became useful precisely because he understood that the villain should not behave like a human villain.
Casting Schwarzenegger also changed Cameron’s original conception of the machine. Cameron later described the earliest idea as an infiltrator whose appearance could be relatively ordinary. Lance Henriksen, who eventually played Detective Vukovich, had been associated with that early direction. Schwarzenegger created a completely different effect. Cameron began seeing him as something closer to a human armored vehicle whose size itself communicated mechanical strength. (BFI)
Michael Biehn’s casting as Kyle Reese strengthened the contrast. Biehn did not look like another action star competing with Schwarzenegger for physical dominance. Reese looked tired, alert, underfed, and vulnerable. His strength came from knowledge and urgency. In a direct physical contest, however, he had no plausible advantage.
The production itself was far from a giant studio spectacle. AFI records that principal photography began in February 1984 with a budget of about $6.5 million. Schwarzenegger’s commitment to Conan the Destroyer had delayed the schedule, and filming lasted about ten weeks, much of it at night in Los Angeles. Cameron did not yet have the resources that later accompanied Terminator 2, Titanic, or Avatar. (AFI Catalog)
That limitation made Schwarzenegger’s performance even more valuable. Cameron could not show the metal endoskeleton continuously. For most of the film, the audience had to believe the machine was inside a human body. Schwarzenegger’s physical behavior became an inexpensive special effect. A weapon handled without thought, an unchanged expression after violence, or a body that kept moving after injury told the audience what expensive effects could show only occasionally.