Talk to me about what fitness has meant to you over the years. When did training become part of your daily routine?

There’s been a fair amount of back and forth. I didn’t get serious about fitness until 15 years ago. I had got very out of shape and my wife introduced me to a woman named Dana Perri. Working with her, I lost 54lb [3st 12lb] in the year I turned 54. And I’ve tried not to look back. I work out with Dana when I’m in LA and I have my guy Charles Cooperman in New York. Now, when I go up and down, it’s just a little bit, instead of completely falling off the wagon.

The range of your body weight is tighter.

Exactly – it’s 6lb or 7lb. Fifteen years ago, I weighed 250lb [17st 12lb] and there was very little muscle involved in that. It was something I had done on purpose for a part. A few years later, I was playing a couple of emaciated characters – one was incarcerated and being starved, and the other was a sad old guy in a movie called I’m Not Here. I worked out with my pal Aaron Williamson and we got me down to 154lb [11st]. I was healthy but scrawny. But the 96lb weight range? Those days are over.

There’s been a lot of talk about the shape you’re in for Red One. How did you prepare?

Well, when you’re reading a script and you see on page 20 that there’s a scene in a gym where you’re working out with The Rock, that’s your motivation right there! It’s just old-school hard work and nutrition. I’ve never gone in for any of the miracles of modern science – you know, ‘chemical help’. It doesn’t seem worth it at my age.

What does 60 minutes in the gym with JK look like now?

Well, now it’s about 60 minutes, but when I was training really hard for the movie, it was more like 120 minutes. It’s moderate weights. You don’t skip leg day, especially at my age. Working on staying strong in the lower half, in the core – all those things. Partially, it’s about trying not to look too pathetic next to someone like Dwayne [Johnson]. Some of it is working on the movie muscles, but a lot of it is just trying to stay healthy.

A few years ago, a gym photo of you went viral: you’re in the arm blaster, you’re curling, you’ve got the big beard. Had you already got the role in Red One, or did that image inspire the casting?

I’d have to ask Dwayne [Johnson], Hiram [Garcia, producer] and Jake Kasdan [director] about that! I was working out with Aaron and I wasn’t training for a specific role. I had just finished I’m Not Here, so I was really, really lean. And then, as we started building muscle again, it was a great time for him to click those photos. The arm blaster doesn’t hurt either. You look like a beast, you know?

My friend’s son was the first guy to use the term ‘shredded Santa’, because I had the beard and I was just about to shave it off to play Commissioner Gordon in Zack Snyder’s Justice League.

But really, my hard work in the gym at that point was just trying to maximise my potential and my health. It had nothing to do with playing Commissioner Gordon, who’s always wearing a trench coat. We have no idea how fit he is.

In the gym with personal trainer Aaron Williamson in 2016.

Does working out help you mentally, too? Do you consider it part of your creative process?

Absolutely. Especially if I’m training for a part where the character’s meant to be in good shape. But just in general, when I don’t make time to work out, I don’t feel as good. That doesn’t mean 90 minutes of hard pumping every day. Sometimes it’s 30 minutes on the exercise bike.

You also cut an impressive shape as Terence Fletcher in Whiplash, which had a theatrical rerelease for its 10th anniversary this year. Does your gym time support your performance in front of the camera, even when it’s not vital for the role?

[When I shot Whiplash], a lot of people still had this vision of me as the guy who had been 50lb heavier. And that was the character Damien [Chazelle, director] had in mind: a bigger, heavier guy, wearing a suit and tie.

I suggested, you know, I’m not that guy any more. I’m a little more fit and I’d like to emphasise that, in terms of being an intimidating figure for these young musicians. He jumped on board. It became part of who Terence Fletcher is.

It was a challenge on so many levels, because Damien had Miles Teller in mind to play Andrew Neiman. Miles has a good three inches on me and is obviously young and fit. Damien had told him not to be in particularly good shape for the part, because he was trying to de-emphasise that.

Your co-star in Red One is, of course, Dwayne Johnson. Did you manage to get in the gym with him?

Only on camera! Working with someone like DJ – who, first of all, is such an open and generous, kind-hearted guy – one of the things I’m sure everyone wants to talk about with him is training and nutrition. I definitely picked up some tidbits.

You also worked with Arnold Schwarzenegger in Terminator Genisys. Do you think Arnie could still hold his own in a workout with DJ?

Well, I haven’t seen Arnie in a few years. But the Arnie that I worked with could hold his own with anyone on the planet.

If you could hit a workout with anyone, alive or dead, who would it be?

Well, DJ, I mean, he’s the guy… Although, I recently had the chance to work out with Clint Eastwood [while shooting Juror #2, out 1 November]. But I got the text too late, ‘Hey, you want to come join Clint in the gym?’ I’m out eating lunch and I could have been in the gym with Clint!

After getting ’really, really lean’ for his role in I’m Not Here, Simmons threw himself into building muscle again.

If you could only do one exercise for the rest of your life, what would it be?

Dead bugs. It’s such a great overall body exercise. I’ve learned, over the years, it’s core, core, core. If you’ve got a strong core, you can protect your back and everything else will follow.

What’s next on the horizon for you?

I’m heading to Iceland in a few weeks to do a movie called Reykjavik, where no one is particularly fit and we all wear suits and ties. It’s about the Reagan-Gorbachev summit in 1986. Lots of sitting and talking, so a bit different from Red One.

Do you find the energy and the vitality you get from being in the gym as often as you are helps you in roles like that, too?

Absolutely, because if you’re just rolling out of bed and sitting in a chair all day, it’s hard to stay mentally alert. But if I get my butt out of bed and get in a nice solid workout, I just feel energised – physically, mentally, emotionally – to get through those long days.

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Scarlett Wrench is the Senior Editor at Men’s Health UK.

With more than 12 years’ experience as a health and lifestyle editor, Scarlett has a keen interest in new science, emerging trends, mental well-being, and food and nutrition. For Men’s Health, she has carried out extensive research into areas such as wellness in the workplace, male body image, the paradoxes of modern masculinity, and mental health among school-age boys.

Her words have also appeared in Women’s Health, Runner’s World and The Sunday Times.