When Matt Damon and Paul Greengrass, the men behind the two best Bournes, reteam for a film, you can expect a certain amount of bang for your buck.

This time though, Matt isn’t an amnesiac assassin criss-crossing Europe trying to discover who turned him into a weapon of close quarters destruction.

He is an army officer criss-crossing Iraq trying and failing to find the scare-mongeringly named weapons of mass destruction that were ostensibly one of the reasons the army was sent in in the first place.

“It seemed like such fertile ground to make a film from, even though we didn’t know exactly what the movie was going to be,” says Matt.

“The fundamental question was could we make a film that would get a good chunk of the Bourne audience over into a story that was about a fictional character in the real world world rather than a fictional character in the fictional world.”

Considerably better looking and more heavily armed than the Chilcot Inquiry, Matt, as Chief Warrant Officer Roy Miller, is the man asking the questions the public want answered – about precisely why they appear to have been duped into a wild goose chase and by whom?

Fictional theorising is blended with some of the more media attention grabbing elements of the real war – such as putting the heads of the ‘most wanted’ on packs of playing cards.

Material was also drawn from the former Washington Post journalist Rajiv Chandrasekaran’s book Imperial Life in the Emerald City: Inside Iraq’s Green Zone, which catalogued how the occupational administration holed up in the luxurious comfort of Saddam’s former palace while ignoring the realities of the suffering outside.

Damon, 39, spoke to soldiers sent in to find the WMDs to understand their feelings and frustrations when they came up empty.

“Monty Gonzales (Chief Warrant Officer 4 Richard Lamont Gonzales), our technical advisor, led a team in comprised of scientists and EOD (Explosive Ordnance Disposal) guys, and people drawn from an artillery division to be security for them, help them get into these places, fight their way in if necessary and protect them while they did their work.

“Monty had all these intel packets and it was the very first factory they went into that he was sure something horribly wrong had happened.

“It was listed as a dual use facility – it was a porcelain factory but was hiding the fact that it really was making something else.

“Monty took one look at it and said there is no way this is anything other than a porcelain factory and anybody who says otherwise is making it up.

“Monty is exactly the same age as me. We graduated the same year. I went to college, he went into the army. He comes from a military family and is very proud of his service.

“I asked him ‘Why are you participating in this (film) experience. What do you hope to get out of it?’

“He said: ‘We have lost our moral authority’.

“That all went to the reasons for going in, for invading a country and being the actual guy who goes into a factory and says ‘This is not what they said it was’.”

The release of Green Zone will come a week after what should hopefully be a crowning moment at the Oscars for director Kathryn Bigelow and multi BAFTA winner The Hurt Locker.

That similarly Iraq-based, tension-laced drama deals with the explosives experts tasked with the hazardous job of ridding the streets of Baghdad of unexploded bombs.

The two films share a cinematographer, Barry Ackroyd, who has been nominated for an Academy Award for his work on Hurt Locker.

“He and Paul set up an environment where there was such freedom (on Green Zone),” says Matt. “There was never a mark that was laid down. Nobody ever said you have to go and stand there and deliver the line this way.

“Their interest lay totally in capturing something in real time.

“They had a back-up camera so when one camera would dump they’d pick up another and carry on. That allowed the actors and non actors, of which there were many, to stay in that world without everybody breaking and going to get a cup of tea or going to the bathroom.”

Matt has also been Oscar-nominated, for his supporting role in the Clint Eastwood-directed Invictus, which was released last month.

He slipped in some publicity for Green Zone while still in London completing his second film for Clint – Hereafter.

A break from his hectic schedule does not seem likely in the near future.

“I am on a flight in a few hours so I am looking forward to getting my head down,” he jokes.

“I just finished with Clint yesterday and that is like taking time off. He shoots no more than 10 hours a day and it is a very civilised schedule.

“I want to direct someday and I can’t really pass up the chance to work with the people I am getting to work with. Paul three times, Clint twice, (Steven) Soderbergh five or six times. So, yeah, I can’t see taking time off unless the work dried up.”

Next on his to-do list (and before a rumoured professional reunion with his friend and Good Will Hunting co-writer Ben Affleck) is starring in a True Grit remake for the Coen brothers.

Jeff Bridges will be stepping into John Wayne’s cowboy boots as ‘Rooster’ Cogburn while Matt takes on the role of Texas ranger Le Boeuf, famously played by country star Glen Campbell.

“The Coens have gone back to the source for True Grit and that is the wonderful book Charles Portis wrote. Most of the dialogue is culled straight from the book.

“So I am relying less on Glen Campbell and more on Joel and Ethan for that performance.

“And there’ll be no singing. You’ll all be spared.”