The ?rst time Laura Poitras was stopped and questioned at an airport, she thought it was a mistake. Flying home to the US from the Sarajevo ?lm festival in 2006, she was paged at Vienna airport and asked to go to security. She was put on a bus, taken to a baggage inspection room and questioned about her trip. She asked: “Why are you stopping me?” The answer: “Well, you know, your name came up on a US government list, and you have a threat score that is really high.”

If it was high then, today it is stratospheric. Poitras has played a key role in the world’s greatest leak of espionage secrets — American whistleblower Edward Snowden’s revelations of a huge US electronic surveillance programme. Poitras is one of only two journalists to whom Snowden last year entrusted his treasure trove of documents taken from the National Security Agency, the cyber intelligence organisation. She is also the director of Citizenfour, a ?lm about her encounter with Snowden, which is tipped for an Oscar as best documentary of 2014.

Poitras now assumes she is under surveillance, night and day. “I am lit up like a Christmas tree behind the scenes,” she says, quite casually. “Which means there is probably a graph, and the graph shows who are the people that I am in contact with.” She is speaking in Berlin, where she now lives. Milky autumn light streams through the windows, gently illuminating her. She looks younger than her 52 years and gestures fluidly when she speaks. If she worries about the perpetual monitoring of her daily life, she does not show it. “The choice is, either I say: ‘Well, I’ll stop doing this kind of work,’ you know, because the harassment is really bothersome, or ‘I’ll keep doing it.’”