From Laura Poitras to Jacob Appelbaum to Sarah Harrison, Berlin has become a haven for American journalists, activists and whistleblowers who fear America's unlimited appetite for surveillance and put their trust in Germany's memory of the terror of the Stasi.

An excellent, long profile of the expatriate scene in Berlin paints a picture of a city where the shadow of the sovietized, suspicion-haunted German Democratic Republic era is cast over the public discourse, creating sympathy for activists who expose spying, and suspicion of the states that would punish them. It's occasioned by the launch of Citizenfour, the brilliant documentary about Edward Snowden and the leaks he took with him out of the NSA.

But then Hubertus Knabe tells me: "The minister of the Stasi always said, 'We have to answer the question, who is who?' Those were his words. That means, who thinks what? It used to be an obvious fundamental difference between a democratic state and a dictatorial one that you don't investigate someone until they did a criminal act. Innocent people are not surveiled. And in this, the difference between how a democratic state acts and how a totalitarian one acts has diminished. And this is very, I don't know the English word. Besorgniserregend? Hold on, I will look it up," and he taps into his phone. "Alarming! This is very alarming to me."

I'm about to leave when he tells me about a conference he held recently at the museum. "And this man, a former prisoner, kept saying this very strange thing. It was very annoying at first. He kept saying, 'I am your future'. 'I already experienced what will be your future.' But he was very serious. He had emigrated to Paris. He really meant it."

The German premiere of Citizenfour is at the Leipzig film festival. It's a town in the former East Germany that's famous for its role in starting what the Germans call "the peaceful revolution", the acts of civil disobedience that led, seemingly out of the blue, to the fall of the Wall on 9 November 1989. And before the screening, an introduction from Edward Snowden to the people of Leipzig is broadcast. "Your history is an inspiration to me," he says. "It is critical to remember the lessons of history." Of how a regime was changed "by ordinary people in the streets".

Having now met Poitras, it's no surprise that Citizenfour is such a quietly humane film. It shows Snowden's courage and conviction but also his vulnerability, his youth; the terrible self-awareness he has of everything he's giving up. Poitras is the softly spoken, self-effacing counterpart to Glenn Greenwald's more strident style of media engagement. It was Snowden who first got in touch with her, and it was her familiarity and facility with encryption techniques and security measures that made the entire story possible. It's not just Snowden who comes across as brave and principled.