Thinking back to the beginning of the project, Gordon-Levitt says his first impression was simply awe at getting to work with Oliver Stone. "When I first got the job offer, I was just excited to get a job offer from Oliver Stone," he laughs. "I'm a fan of his! But I really didn't know very much about Edward Snowden. I had heard his name, but when I asked myself, 'What exactly did he do and why did he do it,' I realised I had a lot of learning to do."

Joseph Gordon-Levitt stars as Edward Snowden

As he read a variety of articles about Snowden, Gordon-Levitt realised that he had to make a decision about how he was going to play him, either as a truth-teller or a traitor. "It's about trying to analyse your source," Gordon-Levitt says. "This person's saying that what he did was a service to the country. This person is saying that what he did is harmful to the country. Who are these different people? Where are they coming from? What is their incentive? Who do they work for? Who pays them?"

And of course he also had the chance to meet Snowden in his Moscow exile. "When I met Ed, I was really focused on him as a human being," he says, "which is funny because he's always trying to take the attention off of himself and put the attention on the issues that he brings up. But because I'm going to play him as an actor, I really wanted to get a sense of what he's like. It was little things like how did he shake your hand, how does he sit in the chair, how does he eat his lunch?"

Gordon-Levitt feels that making the film has given him a new insight into issues of security and privacy. "It's not only the government, there are companies collecting information as well," he notes. "But look, I'm a real optimist, and I tend to see the internet as something that has had, and will continue to have, enormous positive impact on all of our lives. Before working on this Snowden project I never really stopped to think about any potential downsides, and the truth is every technology, especially a powerful technology, always can be used for good or for bad."

Watch the trailer for Snowden: