Last year, UK cinemagoers were treated to two competing accounts of the story of Julian Assange: Bill Condon’s oddly inert drama The Fifth Estate, and Alex Gibney’s more pointedly dramatic documentary We Steal Secrets: The Story of Wikileaks. Although very different in form, content and, indeed, success (Gibney’s film was Bafta-nominated, Condon’s was hailed as one of the year’s biggest flops), both movies wrestled with the conundrum of separating the cult of Assange’s divisive personality from the significance of the information that he helped to publish – for better or worse.

There’s a similar tension at the heart of Citizenfour, which intimately documents whistleblower Edward Snowden’s efforts to lift the lid on the intrusive post-9/11 US eavesdropping industry. Yet unlike Assange (who appears briefly), Snowden shows no signs of wanting to be the centre of any story; on the contrary, he seems positively camera-shy as Oscar-nominee Laura Poitras captures him in a Hong Kong hotel room in 2013, over eight tense days during which his revelations are first made public. Accompanied by the contrastingly gregarious Glenn Greenwald and the Guardian’s meticulous Ewen MacAskill, Snowden attempts to weigh up the importance of standing up to be counted against the possibility of the story becoming about him, rather than his information. With winning candour, he admits that he has no experience of the media and is eager to hand over decisions about what should be revealed, and when, to people whom he trusts to know better – another stark contrast with Assange’s more egomaniacal approach.

Key to Snowden’s trust is Poitras, who earned herself a place on the Department of Homeland Security’s “watch list” with 2006’s My Country, My Country (about life under US occupation in Iraq) and 2010’s The Oath (filmed in Yemen and Guantánamo) and here builds upon her 2012 short The Program, wherein NSA whistleblower William Binney revealed how technology designed to gather foreign intelligence was used “to spy on US citizens without warrants”.

An NSA contractor employed by Booz Allen Hamilton, Snowden became similarly alarmed that he was “designing systems to amplify state power”. Encrypted emails to Poitras led to clandestine encounters of the kind that normally only happen in spy movies, replete with coded exchanges (“Do you know the restaurant’s opening hours?”) in exotic foreign locales. After their secretive sojourn in Hong Kong, during which the bulk of Citizenfour was filmed, the various parties dispersed, with the now outlawed Snowden seeking refuge in Russia, Greenwald returning to Rio and Poitras heading to Berlin to complete her film in secrecy.

The film’s final stanza, which was carefully “redacted” until the New York premiere last Friday, reunites the central characters in Russia, where they share a broken conversation in which key words are scribbled on notepads rather than spoken aloud (for fear of listening devices) and then shredded before the camera’s prying eyes. John le Carré couldn’t have made this stuff up.

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As with so much of her previous work, Poitras’s documentary dramatically straddles the divide between art and journalism, cinema and reportage. The trailer for Citizenfour, which warns that “every border you cross, every purchase you make, every call you dial... is in the hands of a system whose reach is unlimited but whose safeguards are not”, plays it as the most chilling thriller of the year, and that’s no an idle boast; there is tension here to rival any of executive producer Steven Soderbergh’s taut dramas.

Outside of the interviews, Poitras presents us with lines of dot-dash lights that could be digital code, but gradually come into focus as the overhead illuminations of a long, dark tunnel – the rabbit hole down which we must travel. An ominous, rumbling score adds menace, suspended chords and electronic creaks suggesting a descent into some Stygian world. Even in the brightly lit hotel rooms, the sound continues to alarm, the shrieking bell of a suspiciously timed fire drill setting our subjects’ nerves on edge – and ours.

Revelations that every phone and mobile device can be remotely used as a bug (at one point, Snowden hides beneath a cowl to protect his keystrokes) crank up the tension further still; when a mobile phone went off in the early screening I attended, the audience’s annoyance was tempered by all-too-nervous laughter. I’m sure I wasn’t the only one who went home and took the battery out of their mobile.

At times I was reminded of Jafar Panahi’s This Is Not a Film, which found the Iranian auteur covertly making movies while under house arrest in Tehran. Elsewhere, there are echoes of The Internet’s Own Boy and the youthful altruism of Aaron Swartz; one sequence in which Snowden attempts to disguise himself via the application of hair gel would not look out of place in a John Hughes movie, his now familiar tousled cut, specs and preppy smile suggesting a wide-eyed naivety at odds with his apparent understanding (and acceptance) of the magnitude of his actions. It’s a moment of light relief amid a movie of brooding darkness – bravely made and utterly engrossing.

• Comments have been reopened to time with this film’s Australia release