Before he was The Terminator, Arnold Schwarzenegger was approached by James Cameron to play Kyle Reese and actor Lance Henriksen was considered for the title role as the deadly T-800. Cameron even went so far as to draw sketches of what Henriksen would look like as the Terminator in his original concept art for the movie. The studio had other ideas and wanted retired NFL superstar and at the time, Hertz rental car commercial star, to portray the role of the cyborg assassin. Lance Henriksen and Simpson obviously did not end up with the part, but Arnold Schwarzenegger did, and it launched his career even further than Conan the Barbarian had.
While developing The Terminator, James Cameron originally envisioned the character being able to fit into a crowd and get into places without being noticed. Lance Henriksen fit the part perfectly and Cameron even began to sketch up what the actor would look like as the T-800 after an interesting audition. When auditioning for the role, Henriksen put foil on to his teeth and kicked down the door of where the auditions were taking place, scaring the hell out of everybody. Though Lance Henriksen didn't end up with the title role, he did get the part of Detective Vukovich.
The Terminator reportedly came together from a fever dream that James Cameron had where a cyborg walked out of fire. He later said that horror legend John Carpenter was the original inspiration for the style of the movie because of the way he was able to craft something stylish on a small budget, which is pretty funny when one thinks of Cameron's movie budgets now. You can definitely hear the John Carpenter influence in the synthesizer-led score from The Terminator, which was done by Brad Fiedel.
In addition to O.J. Simpson as The Terminator, the studio also wanted the character to have a cyborg dog. The studio apparently wasn't happy with what James Cameron had been working on and left him a few notes. One of which was to make the relationship between Sarah Connor and Kyle Reese stronger and the second was to add a cyborg dog for the T-800. Cameron agreed with the first idea and then decided to quietly ignore the idea of cyborg dog sidekick, which was definitely for the better.
While Arnold Schwarzenegger was cast as The Terminator, he was initially reluctant to take on the role after playing the hero in Conan the Barbarian. James Cameron ditched his idea about having Lance Henriksen in the part as a guy who could fit in to having the muscle man Schwarzenegger take on the role after he suggested how the T-800 should act. Cameron was then successfully able to persuade Schwarzenegger to take on the villainous role and the rest is history. You can check out the original concept art for The Terminator featuring Lance Henriksen as the T-800 below, courtesy of the Core of Movies Twitter account.
An earlier concept-art by James Cameron depicts Lance Henriksen as The Terminatorpic.twitter.com/huKqOwthmu— Core of Movies (@CoreofMovies) March 2, 2018