What Cameron understood was that the Terminator did not need conventional dialogue to dominate the film. The machine’s presence was the performance. Schwarzenegger’s body could communicate enormous strength, but only if the actor refused to use it like a normal action star. He could not swagger after a victory, grimace before firing, celebrate a kill, or show the audience that he knew he looked powerful. The contradiction was the key: one of the most physically conspicuous actors in Hollywood would have to perform as though he had no ego at all.
That idea eventually persuaded Schwarzenegger.
Michael Biehn would later be cast as Kyle Reese, giving Cameron the leaner, vulnerable, exhausted human soldier he had wanted. Schwarzenegger would take the role he had spent the lunch analyzing. BFI records the final principal trio as Schwarzenegger as the Terminator, Biehn as Reese, and Linda Hamilton as Sarah Connor. (BFI)
The change was far larger than a simple swap in casting. Cameron had originally imagined the Terminator as a machine that could blend in. Schwarzenegger made another interpretation possible. Now the machine could look visibly overwhelming, and the suspense would come not from wondering whether it was dangerous, but from realizing that Sarah and Reese could see exactly how dangerous it was and still had almost no way to stop it.
The role was now Arnold Schwarzenegger’s.