Matt Damon on Bourne: Getting into shape was a full-time job

Matt Damon is a low-key movie star.

No entourage, no high-profile model-actress wife, no druggie kids, no scandal. He just puts his head down and works in really good movies.

The 45-year-old US actor is also a smart, thoughtful, socially conscious guy, with a sense of humour, in full display at The Cosmopolitan hotel in Las Vegas, where he is plugging his latest movie Jason Bourne, which opens here tomorrow.

It has been nine years since The Bourne Ultimatum, the last instalment of the action thriller franchise, but the character is still, arguably, his most iconic role.

Jason Bourne is Damon's fourth go-round as the spy who suffered from memory loss and attempted to discover his true identity amid a conspiracy within the Central Intelligence Agency (CIA).

The story is timely, dealing as it does with cyber privacy and the role of the government in a post-Snowden, post-WikiLeaks world.

Bourne comes out of the shadows when he learns that the whole reason he joined the CIA upon the death of his father is based on a lie.

Tommy Lee Jones plays the head of the CIA and Alicia Vikander is the protege he mentors at the agency whose motives are obscure.

The action, as before, is non-stop as the plot shifts from the Canary Islands to London, Berlin, Washington D.C. and Las Vegas, with several thrilling chase scenes all practically filmed with director Paul Greengrass' signature hand-held cameras.

Returning to such a physical role after almost a decade, notwithstanding the ripped bod he displays in the movie, was not easy for Damon.

The father of four says: "The mentality was really easy for me to slip into. The one thing about working as much as I have for the last 20 years is, I did get better. So it's easier to find the character and feel connected to him.

"The physical part was a whole other thing and that was really tough - at 45. I was 29 when I started shooting these movies and it was a very different thing.

"So it was a lot of work. It was two workouts a day and a very specific diet. And there are no shortcuts to getting in shape like that. It's kind of a full-time job."

Will there be another Bourne film? Not right away, according to Damon.

"As Paul always says, the worst time to consider making another Bourne movie is right after you finished a Bourne movie.

"So he needs to go on holiday. And he needs to make at least one other film and maybe more. Then I will gingerly approach him about it, whether we're going to do it again.

"But we're both open to it. We love the franchise and I'll play Jason Bourne as long as Paul wants to direct the films," he says.

BEST BUDDY

Since Damon's best buddy, US actor Ben Affleck, is directing the next Batman movie and the two Oscar-winning writers have not worked together in a while, the question arises whether he would ask for a role in that film.

"Well, if he gives me a job, yeah. People keep asking, 'Are you going to work with Ben?' And I say, 'He's a brilliant director, I would work with him in a second'.

"The big problem is that when he goes to direct a movie, he always gives himself the best role. So, until he breaks that habit, you know, none of us can work with him," he jokes.

CLOSE: US actor Matt Damon, who plays the titular spy in Jason Bourne, with wife Luciana Barroso. PHOTO: AFP

The family - Argentinian wife Luciana Barroso, their three daughters and a step-daughter from his wife's previous marriage - always goes with Damon on location.

He says: "We're very tight-knit. We travel together and we spend a lot of time together.

"And so the countries change and the continents change, but the constant for the kids is each other and their parents. And that's how we're doing it so far.

Damon adds about his companion of 11 years: "I think I got lucky with who I chose. Marriage is incredibly difficult. It's so hard to have that one partner who is everything.

"And, objectively looking at it, it seems like an insane idea. So it's not that I love marriage in general, it's that I love being married to her, and that's the difference."