8. Arnold Schwarzenegger Over Lance Henriksen (The Terminator)
When James Cameron was originally conceiving his 1984 sci-fi classic, he envisioned the titular infiltration machine as a fairly regular-looking human, and planned to cast his friend Lance Henriksen in the part.
Cameron even had Henriksen dress up as The Terminator and show up to the meeting where Cameron was selling the movie, and lo and behold, it worked. The director eventually changed his mind, however, upon meeting Arnold Schwarzenegger, who auditioned for the role of Kyle Reese but was clearly a perfect fit for the cyborg antagonist.
It wasn't a total bust for Henriksen, though: he was still cast in the supporting role of wise-cracking cop Hal Vukovich, and he went on to play the iconic part of Bishop in Aliens for Cameron two years later.
Henriksen probably could've pulled the part off if Cameron had stuck to the idea of the T-800 being a sleek, unassuming unit that could blend into a room, but he likely wouldn't have ever been as iconic as Schwarzenegger's statuesque, truly robotic performance. Everything worked out for the best, basically.