It has been an up-and-down few years for Australian actor Jai Courtney. But we don’t mean his career, which has been on a steep upward trajectory since his breakout role in 2010’s TV cult classic Spartacus: Blood And Sand. No, we mean the number on his bathroom scales, which hasn’t stood still as Courtney changed his body shape dramatically for each little and big screen role. He went from ripped Ancient Roman in Spartacus to super-lean for his portrayal of time-travelling soldier Kyle Reese in 2015’s Terminator: Genisys. And now he’s packed on serious muscle mass for his role as Captain Boomerang in DC Comics’ summer super-villain blockbuster Suicide Squad, starring alongside Will Smith, Jared Leto and Margot Robbie, among others. We caught up with Courtney, and his personal trainer Brendan Johnston, to find out how he got into superhero shape.

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Have you always been into training?

I was always pretty athletic as a kid. As an Aussie I grew up playing rugby league, cricket, swimming – so sport was always a big part of my upbringing. As I got older that led into a lifestyle of living healthy and keeping active and I have always liked training, but I’ve always believed that having balance is a big part of healthy living. As I get older – I’m 30 now – I certainly love having a bit of an off-season, if you know what I mean!

You got into great shape for your role in Spartacus. How was that?

Well, I knew that from the very first day of filming all I’d be wearing was a bloody loincloth! So it was the first time I had taken my training seriously, rather than a bit of fun. It really opened my eyes to the science behind training and nutrition and how you can transform your body to better portray a character on screen. It was really interesting for me to go through that process and I learned so much. But I was younger and could get away with a lot more. It’s remarkable how things do shift and you can become more prone to injury and need to treat your body a little more carefully. But it also allows you to evolve the way you do things to get the right results. You can try different ways of training and new approaches to eating to find the right way that works for you.

How hard was it to get so lean for Terminator: Genisys?

I’m naturally a bigger-frame guy and it’s meant that I could always get away without doing an awful lot and still look in reasonable shape. When it came to Terminator, put simply, they told me I needed to drop a bunch of weight, about 12kg. That wasn’t something I’d ever had to focus on before. They wanted me to get away from being thick and muscle-bound and get really stripped down. So it was a totally different approach. I couldn’t lift any weights and my training was all low-impact cardio and a more restricted dietary regime than anything I’d ever done before. Honestly, it was pretty mind-bending.

What is the key to a successful physical transformation?

It’s cracking the mental side of it, mate, 100%. You have to find relaxation and reward in what you are doing. If I’m working towards a physical goal, it’s amazing how negative the effect of stressing out about achieving that goal can be. It can shift your progress backwards in a major way. You need to be goal-oriented and clear about what you want to achieve in a realistic timeframe. It’s not healthy to assume that you can really change your body in a couple of weeks. Give yourself a couple months and that’s another story – you can see massive results.

How did you train for Suicide Squad?

The beauty of this movie was that I was told I could finally gain some weight! So I went from about 190lb [86kg] in Terminator to 230lb [105kg] for Suicide Squad. So for me the physical preparation was massive. We’d all train together. We did martial arts training together from the very beginning. Will Smith and I would roll on the mats doing jiu jitsu, I was lifting weights with Joel Kinnaman, and Jay Hernandez and I got the gloves on every day and we’d spar. There was a great group mentality. It was really cool. Yeah, it could get a little hot sometimes… but it was always cool, man, because he’s your brother. It’s great when there’s expectation across the board and everyone works together to reach physical goals. You use each other for some camaraderie and healthy competition to push each other.

Was it better training in a group than on your own?

Yeah. It’s competitive, not in that you’re looking sideways and thinking, “He’s doing better than me” but in a way that spurs you on positively to keep pushing. You want everyone to be doing their best. If you can lift with a buddy so he can spot you and push you harder so you hit bigger numbers, then that’s really cool, man. It keeps you fired. Team spirit, a sense of camaraderie, it’s a beautiful thing and as someone who played a lot of team sports growing up, I loved it. One of the best things about training is that you can do it alone, but you can also do it with your mates, teach each other to be better and have a lot of fun in the process.

Who was the strongest member of the Squad?

I can say me, right? Why not? I got pretty fucking big! Will Smith might not be a spring chicken anymore, but he’s in very good shape. There’s a man who knows how to look after himself. He’s strong, man. We were rolling on the mats in the gym and I think he nearly put me to sleep once or twice!

Is doing your own stunts a pro or con of the job?

I love the stunts, man! It’s a lot of fun and I love that side of acting. The most physically demanding work I’ve done was the fighting scenes in Suicide Squad. There’s a sequence at the end of the film that was tough for all of us. We were put under some conditions we had to maintain for about a week – and it wasn’t comfortable! So it’s not always the physical element of a stunt or scene that make them so hard – sometimes it’s the environment you’re in that takes them to another level. You’ve got to stay healthy and do things safely. It’s the only way. It’s easy to drop the ball if your mind wanders if you’re tired, or under-nourished, and that’s when you get hurt.

Suicide Squad is released in cinemas 5th August

NEXT: The Captain Boomerang Workout and Nutrition Plan