JK Simmons final Instagram/@aaronvwilliamson
By now, you have probably seen the Instagram photos of a bearded J.K. Simmons in the weight room flexing a pair of big and powerful guns.

The photos were posted by his trainer, Aaron Williamson, a Marine veteran who has become a trainer to the stars. He has been working with the 61-year-old Simmons off and on since they met on the set of " Terminator Genisys." Business Insider reached Williamson by phone this week to get the scoop on how Simmons has accomplished his dramatic body transformation.

Simmons has been training his body for a while, Williamson told Business Insider.

"When we first trained on 'Terminator' he just wanted to stay healthy, and it progressed after people saw him transform for 'Whiplash,'" he said. Simmons worked with another trainer, Dana Perri, for his role in "Whiplash," but since January, Simmons has begun training again with Williamson as well, usually two to three times a week.

After "Whiplash," Williamson said, Simmons "wanted to do more of a physique transformation."

"He wants to have some 'beefing' guns to show people that he is just not that guy that does Farmers commercials," Williamson said.

So what's the secret to getting this impressive new physique?

"Each session is different — I don't pre-plan any workout before he gets there," Williamson said. "We never go in and do the exact same workout. It is always different." He added, however, that each session followed a relatively similar pattern.

"Usually [Simmons] always rides his bike to gym, so he is getting cardio before he gets there, and then once we meet up we do about 15 minutes more of cardio, then jump into some warm-up sets of whatever body part we are working on just to get some blood in the muscle," the trainer said. Then they jump into working sets — though these are more like supersets.

"I'm big on hitting the muscle from as many different angles as you can with as much blood volume as you can," Williamson said. All these different angles confuse the muscles, creating definition. "We use different exercises back to back to back … [and] by the end of the set, we are essentially doing 30 to 40 reps."

Simmons can do high numbers of reps by using lower weights, Williamson said, though the low weight can still yield results.

For example, Simmons does a lot of dumbbell work, which allows him to change his hand placement throughout the workout, keeping ligaments and tendons safe while working the muscles in his arms in a variety of different ways.

He also does cable work to build up his chest, plank work to strengthen the core, and crunches and leg raises to work the lower body.

There has been a lot of speculation that Simmons is working to transform his body for his role as Commissioner Gordon in the " Justice League" film, but if he is, he hasn't told his trainer.

"If he is, I have no idea about it," the trainer said. "Honestly, when I posted those photos up on my instagram, we never could have imagined this was going to happen and people would assume it was just for this role. But like I said, if he is, he has been pretty hush-hush about anything he is doing. This has just been a personal goal of his to transform his body into something different."