Both films won major awards, but the United States government put Poitras on their security watchlist. She says she was picked up for questioning at the US border about 40 times between 2006 and 2012. Her laptop and camera were confiscated, and she was threatened with arrest when she started to take notes. The border searches stopped only after she contacted Glenn Greenwald, a blogger for salon.com and later The Guardian who is also a former litigator with a strong interest in privacy and human rights issues. Edward Snowden, left, and Glenn Greenwald in the documentary Citizenfour. In fact, Snowden contacted Greenwald before Poitras, but Greenwald became annoyed with all the security measures demanded by the anonymous source. Poitras was already making a film about surveillance and privacy and had just moved to Berlin to try to keep her footage secure from the US government. All of this is a way of introducing a simply extraordinary film by Poitras, documenting the process by which Edward Snowden became, in the middle of 2013, one of the greatest whistleblowers in American history. Citizenfour is not just about Snowden's main disclosure, that the NSA began spying on its own citizens in the week after September 11, despite repeated official denials. It is also about the process by which Snowden's story was constructed and released by Greenwald and other reporters. She shows us the extraordinary secrecy around the process of interviewing Snowden. In doing so, she dramatises the crisis in politics and media concerning privacy. That is one of the film's major achievements: it is an eloquent defence of why privacy matters at a time when surveillance capability has never been greater, nor more unchecked.

Greenwald, Poitras and Ewen MacAskill, the defense and intelligence correspondent for The Guardian, finally meet "Citizenfour" in a hotel room in Hong Kong in June 2013, after elaborate security measures. At that stage, MacAskill does not know even his name. Over the next week or so, Snowden takes the reporters through the documents, explaining his motives and what the documents revealed. Poitras contributes narration in the film but we never see her. Snowden, looking increasingly haggard, is beyond careful. He unplugs the hotel phone while they're talking and puts a cloth over his head and hands when logging on, in case someone else is secretly filming them. The reporters know that he plans to out himself when their stories start running in The Guardian and The Washington Post. No one in his family – not even his partner Lindsay Mills, at home in Hawaii – knows where he is or what he's doing. Poitras films a meeting between Snowden and two human rights lawyers, who will organise his escape from the hotel a few days later. There's footage from within the Ecuadorian embassy in London as Julian Assange helps with the flight that takes Snowden to Moscow, where he claims political asylum. Washington Post during Watergate. But Poitras also paints a bigger picture, with the stories of supporting characters such as William Binney, a crypto-mathematician who worked for the NSA for 32 years. He designed much of the infrastructure the NSA then turned on its own citizens, and the rest of the world, after September 11. He resigned that year in disgust. Citizenfour, like My Country, My Country, is nominated for an Oscar for best documentary. Poitras contributed to the reporting that resulted in The Guardian and The Washington Post being awarded the Pulitzer Prize last year for public service, after their NSA disclosures. Making this film took enormous courage. I watched with feelings of outrage and dread, but also admiration.

Poitras has made a new kind of film – a documentary that reveals its own process, as it explores its difficult subject. In a sense, Poitras and Greenwald are doing what the NSA does, gathering secret information. The difference is they make what they find public. On twitter: @ptbyrnes