Joseph stars as Edward Snowden, the government contractor who in 2013 leaked classified information detailing the workings of America's National Security Agency (NSA), in Oliver's new movie.

And in order to get to know each other better actor and filmmaker decided to smoke cannabis and watch a war film and talk about Oliver's time serving in the U.S. army during the Vietnam war.

"We watched a movie together, a great war movie actually," Joseph told U.S. late night TV host Stephen Colbert. "The first time Oliver and I got stoned together we were talking about war stories.

"I asked him about Vietnam and he started telling me about it and then we started talking about war movies."

Oliver drew on his experiences during and after the war to make two of his most acclaimed films, Platoon and Born on the Fourth of July, both of which detailed U.S. army veterans' experiences of the conflict.

The filmmaker was not the only person on set who Joseph smoked weed with as he also relaxed with an ex-computer hacker hired by producers to ensure the footage Oliver had filmed was safe from cyber-attack.

"There was a guy on set his name was Ralph, who was sort of our technical consultant. He actually used to be a hacker, he's an extremely sophisticated hacker but now he works sort of on the other side," the star said, adding, "Ralph also happened to have the best weed."

Snowden is in exile in Russia and the runner-up in the race to become Democratic presidential candidate Bernie Sanders is leading a chorus of prominent public figures calling for clemency, a plea agreement or, in several cases, a full pardon for the National Security Agency whistleblower.

Snowden, which also stars Shailene Woodley, Zachary Quinto and Melissa Leo, debuts in cinemas on Friday (16Sep16).