J.K. Simmons stunned the Internet when his trainer ... the on-set trainer for Terminator Genisys. "I just had free time to go to the gym, so I started training with him," Simmons says.

He first got his hands on Simmons in 2015 to help him shape up for Terminator Genisys ... he had ever been in," Williamson says, adding, "J.K.'s goal was to adopt a healthy lifestyle and ...