who did the studio orion originally want to play the t800 in the terminator
Sylvester Stallone and Mel Gibson
The Terminator
Sylvester Stallone and Mel Gibson were offered the Terminator role, but both turned it down.
wiki:19647293
The Terminator [SEP] Pre-production
For the role of Kyle Reese, Orion wanted a star whose popularity was rising in the United States but who also would have foreign appeal. Orion co-founder Mike Medavoy had met Arnold Schwarzenegger and sent his agent the script for "The Terminator". Cameron was dubious about casting Schwarzenegger as Reese as he felt he would need someone even more famous to play the Terminator. Sylvester Stallone and Mel Gibson were offered the Terminator role, but both turned it down. The studio suggested for the role but Cameron did not feel that Simpson would be believable as a killer. Cameron agreed to meet with Schwarzenegger about the film and devised a plan to avoid casting him; he would pick a fight with him and return to Hemdale and find him unfit for the role.