Years later, Schwarzenegger summarized the idea this way: “He’s a machine. So everything has to be matter-of-fact ... no joy, no gratification ... Just the mission, complete.” (Men's Health)
That was only the beginning of his analysis. Schwarzenegger argued that the Terminator should handle weapons without looking down at them because a machine would already know exactly where every component was. It should be able to strip, reload, and manipulate a gun with motions that looked programmed rather than improvised. When it fired, it should not blink. Explosions should not surprise it. Killing should produce no anger, pleasure, satisfaction, or adrenaline. The machine should simply finish one task and continue to the next. (Men's Health)
This was not how Cameron had expected the lunch to go. Schwarzenegger was not lobbying for the villain. In his own recollection, he still wanted to play Reese. He was simply so interested in the Terminator that he kept explaining how the part should work. Cameron later remembered noticing that the scenes Schwarzenegger quoted enthusiastically were Terminator scenes rather than Reese scenes. While Schwarzenegger talked, Cameron began studying the man sitting across from him differently. (BFI)
The physique was obvious, but Cameron has said it was Schwarzenegger’s face that struck him most strongly. He began imagining that face absolutely still, with the actor’s size no longer belonging to a heroic barbarian but to something mechanical. The Terminator had originally been conceived more like an infiltrator: a threatening figure who could disappear into a crowd rather than an enormous walking weapon. Cameron had even imagined his friend Lance Henriksen as a possible model for that direction. Sitting across from Schwarzenegger, he saw another version of the character—a human-looking machine whose sheer physical presence made the audience understand immediately that ordinary force would not stop it. (BFI)
That realization solved one casting problem while creating another. Schwarzenegger was clearly wrong for Cameron’s conception of Reese, but the qualities that made him wrong for the hero could make him extraordinary as the Terminator.