In The Terminator, Lance Henrikson was originally supposed to play the machine and Arnie was going to be Kyle Reece, but it was Arnie that convinced James Cameron that he should get the role of the machine.
You read it correctly there was a movie concept in 1990 for Spiderman with Arnold as a villain but producers were less than impressed. And James Cameron was writing the script!
In The Terminator, Lance Henrikson was originally supposed to play the machine and Arnie was going to be Kyle Reece, but it was Arnie that convinced James Cameron that he should get the role of the machine.