Back in 1984, Arnold Schwarzenegger was already a very recognizable actor. However, "actor" was seen as more a technicality of his job than his talent and capabilities. This outlook changed quite sharply when it was decided that instead of playing the hero of horror movie reviews this time-travel-killer-robot-dark-future thing James Cameron had been working on, he would play the villain. That (blog post) meant instead of just being strong and yelling (blog post) a lot, he had to portray a killing machine.. which it turns out he's very damn good at.
Source: The Terminator Movie Review - Decker Shado