J.K. Simmons' Trainer Reveals How the Star Got Huge Ahead of 'Justice League'

Aaron Williamson, a Marine-turned-trainer, tells THR the secrets to the 'Whiplash' star's impressive physique: "His goal was to have 'sick arms.'”

If you were clicking around the internet on Wednesday, you were likely stopped dead in your tracks by a photo of a bearded J.K. Simmons in a weight room flexing a pair of guns so big and powerful they could make an N.R.A. member blush.

The startling shot of the Oscar-winning Whiplash star was posted to Instagram by his trainer, a 6-foot-3, 235-pound former Marine and competitive bodybuilder named Aaron Williamson. The Hollywood Reporter caught up with the 36-year-old Williamson to find out how the 61-year-old Simmons achieved such impressive gains to his manly physique — and whether this all has something to do with his upcoming turn as the police chief of Gotham in Warner Bros.' Justice League.

Besides J.K. Simmons, I see you training Dwayne Johnson and James Marsden on your Instagram feed. How did you amass such a starry clientele?

It’s just a destiny thing. I came back from Iraq in 2009 and I was dealing with some pretty rough stuff in my head with PTSD and job opportunities that never panned out. So I started focusing on fitness training and from there fell into the middle of the movie industry. At the end of 2010 is when I started training people in the industry in New Orleans. I became the go-to guy there. I just moved to Los Angeles last September.

What were some of the big films you've worked on?

Terminator Genisys, Fantastic Four, G.I. Joe: Retaliation, Django Unchained. I was training the actors for a lot of them but then I started getting into acting. G.I. Joe: Retaliation was my first time in front of the camera. Terminator: Genisys was where I met J.K. I was training the cast and I play the refugee Terminator in that film. This was before Whiplash.

What were his fitness goals at the time?

For Terminator, he wanted to just keep himself healthy while filming. And somehow during the course of him getting ready for Whiplash he wanted to take things to the next level. He has another trainer in Los Angeles. She’s not the same type of trainer that I am, so we complement each other. He started doing this cardio strength-training class with her and dropped a lot of weight. He started to fall in love with the fitness lifestyle. Where I come into play is now that he’s at this point we are giving him muscles he didn’t have before. His motivating factor with a lot of it is to do something with his arms and shoulders to be able to possibly down the road play a character who can wear a tank top.

"Possibly down the road?" Based on that photo you posted to Instagram he’d probably tear any tank top you put him in.

(Laughs.) When you look at him in the gym, people are literally doing double-takes. They can’t believe that’s him. He’s doing such a good job of being disciplined and staying dedicated to doing something he hasn’t done before with his physique. He’s ripped. He’s more shredded than I am.

He's next playing Commissioner Gordon in Justice League. Was this something that director Zack Snyder asked of him?

As far as the whole Justice League stuff goes, if he is trying to get ready for something in that arena, I’m not aware of it and he never shared it with me. I’m only training with him as a friend and client and helping him achieve what he’s trying to achieve with his new physique.

Which Gold’s are you training at?

We switch it up a lot. The one in the photo is the North Hollywood Gold’s.

How often?

About two or three times a week.

Are you giving him a specific diet?

I give him suggestions here and there, but he’s been experimenting over the past couple of years with different foods. A lot of low-carb stuff, lean proteins, a lot of vegetables. Every day might be a little different for him. He eats every three hours. What he calls his “secret weapon” is a little piece of dark chocolate. He eats one square every day.

Literal chocolate or "human growth chocolate"?

Literal. Just a good, high-quality piece of dark chocolate. Whenever we go out for lunch, he’ll pull one out for me. I say go for it. There’s a point that if you don’t cheat, your metabolism will actually stall out.

Have you seen Whiplash?

Yeah.

Does he, uh, transform into Terence Fletcher, Incredible Hulk-style, in the weight room?

No, he’s literally the nicest guy I’ve met in L.A. — the most down-to-earth, genuine guy you can possibly be around. I know it’s cliche to say about someone “giving you the shirt off their back,” but that’s what he is.

What can he bench?

We really typically are doing a lot of unilateral movements with dumbbells, because J.K. has aches and pains and stuff, so we try to keep things mobile and loose. We use dumbbells that feel a bit better on your joints rather than being stuck under a bar. His goal was to have “sick arms.” When we did Terminator, I was training a guy named Jai Courtney. He kept mentioning, “I want to have arms bigger than Jai’s.” We would joke about it, but then we actually started training and he made improvements on his arms until what we’re doing right now.

Does all the focus on the arms run the risk of rendering him a little top-heavy? We all know how muscleheads love to mock the guys that skip legs day.

In that one picture that’s going viral, we’re using an arm-blaster. That piece of metal behind his arms pushes up on your triceps so it’s going to puff your arms up. But you’ll see another picture of my Instagram where he’s doing a fly. His arms are not as big in that one — but they are completely shredded.

And here I thought he was dressed like Flash Gordon.

Arnold Schwarzenegger would appreciate this because the arm-blaster became famous back in the golden era of bodybuilding. It’s a really cool piece of equipment. You don’t see a lot of gyms with it.