OJ Simpson was originally cast to play Terminator, but director James Cameron did not feel that Simpson would be believable as a killer.
Originally, Cameron and Hurd planned to have Lance Henriksen play the Terminator. However, Orion Pictures President Mike Medavoy pitched Schwarzenegger as the "name" actor, but for the role of Kyle Reese. For the Terminator, he suggested none other than Orenthal James "O.J." Simpson, former star NFL running back and, at the time, perhaps best known for his popular series of commercials for Hertz Rent-A-Car.
Medavoy saw Simpson not only as a "name" (he had been in a few movies by that point) but also, more importantly, as an athletic person who could be physically imposing. Medaovy even approached Schwarzenegger at a screening and told him Simpson had been cast and, that the studio wanted Schwarzenegger as Reese.
Schwarzenegger talked with Cameron about playing Reese, although the director had gone into the meeting hoping the conversation wouldn't lead anywhere. Instead, he found himself impressed by Schwarzenegger, especially his insightful ideas about how the Terminator should come across on film. Cameron liked him so much that he suddenly decided the Terminator should be a really big guy, someone like, well, Schwarzenegger.