Oliver Stone has revealed he used encrypted messages to communicate while making his film about whistleblower Edward Snowden, due to fears of hacking.
The drama – called simply Snowden – looks at how the former CIA employee leaked details about the National Security Agency in 2013.
It is due to be shown at the London Film Festival next month.
Speaking at the movie’s premiere in Toronto, the 69-year-old movie director said: “The NSA is worldwide, the ability to intercept, to harm.
“Anything could happen. We’ve still not opened the film. It could get hacked tomorrow.
“We lived on our nerves. We stayed off grid completely.
“We really tried to do everything in person, on paper, and encrypted as much (as) we could.
“We encrypted all long-distance communications.”
The double Oscar winner – best director for Platoon and Born On The Fourth Of July – said it was “not only eavesdropping” that Snowden had brought the world’s attention to, but “cyber warfare and drone attacks too”.
Meanwhile Zachary Quinto, who plays journalist Glenn Greenwald in the movie, says Snowden should be allowed to return to the US without facing espionage charges.
The actor, known for playing Spock in the rebooted Star Trek films, said: “I think it’s a very complicated issue in terms of how that would happen.
“The idea of him being charged under the Espionage Act or branded as a treasonist is absurd.
“I think he is someone of great integrity and great courage.
“I think what he did is underestimated now, in a lot of ways, but I think (it) will be looked back on with the magnitude it deserves.
“Hopefully he can enjoy some freedoms again in his life. He deserves that in my opinion.”
Snowden, who has been living in Russia since June 2013, has said he would return to the US if he was guaranteed a fair trial.
He is portrayed in the movie by Joseph Gordon-Levitt, who said he had a “valuable” four-hour meeting with him in Moscow.
Speaking to reporters at the premiere, Gordon-Levitt said: “In reading up on him and really learning about what he did and why he did it, I felt grateful for what he did and honoured I got to play him.
“I don’t think a single label is appropriate. Everyone tries to simplify this story.
“It’s not simple, it’s complicated.”