Knoxville made his name playing outrageous practical jokes on MTV series Jackass, but The Terminator star warned him there would be consequences if he tried any of his gags while they shot their new blockbuster.
Schwarzenegger tells Britain's The Sun newspaper, "I forbade him. I told him the payback would be very painful.
"Johnny is used to Jackass films, where the more something goes wrong and doesn't work, the better. I said to him our movie was different and he had to change his attitude."
However, Schwarzenegger is adamant Knoxville was a great addition to the cast, adding, "He was really good fun to work with - he brought some good comic relief to the movie."