'Risk', Laura Poitras's portrait of Julian Assange, the WikiLeaks mastermind, has been compared with her 2014 Oscar-winning 'Citizen Four', about the former National Security Agency contractor and whistleblower Edward Snowden. Both films benefit from the extraordinary access the director had to her subjects, but they differ in crucial ways.

'Citizen Four' has a clear dramatic focus, circumscribed timescale and straightforward citizen-hero in Snowden. 'Risk' is much more slippery and open-ended, matching the ambiguity of its central character as well as the film's more extended time frame: 'Risk' was filmed over six years, from 2010 to 2016, with most of the interviews conducted between 2010 and 2013 when Assange and WikiLeaks seemed to be in the vanguard of a global movement for openness and democracy.

The most crucial difference, however, lies in Poitras's attitude to her films' subjects. In 'Citizen Four' she takes Snowden at face value. In 'Risk', as Poitras gets closer to Assange and his team over time, the mood darkens and her perspective shifts. Initially motivated by feelings of comradeship and admiration ('I thought Wikileaks was doing the hard journalism that hadn't been done for a long time post 9/11… It was crucial and brave journalism') she ends up more distanced and critical.

We still see the man of principle courageously exposing secret institutions that shape our lives, but we also see the pompous, narcissistic and sometimes paranoid Assange whose main concern seems to be his own fame. The end result is a film whose structure is much less straightforward than it might seem. It is also a film that Assange and his team hate.

Poitras presents the WikiLeaks story in verité style. Apart from some conversations between Assange and the director, with Poitras always off-camera, there are no interviews and no commentaries. Poitras occasionally inserts her own thoughts from her diary and some atmospheric music by Jeremy Flower to heighten the drama, but this intimate portrait of the man and his organisation reveals rather than probes. By fixing her camera's sights on Assange and his circle and seldom widening the frame to bring in a broader context, the result is a claustrophobic, gripping and intriguingly unresolved piece of cinema.

Poitras began filming Assange in his grand hideaway house, Ellingham Hall in Norfolk, in 2010. WikiLeaks had already released thousands of classified documents concerning the US war in Iraq. We witness a constant flow of admiring young women and some men catering to Assange's every need, working on WikiLeaks plots, fussing over him as he dyes his hair and inserts contact lenses for an incognito motorbike turn round town, and eating large dinners. Of one of these meals, Andrew O'Hagan, writing in the London Review of Books about his abortive ghosting of Assange's memoir, reveals that Assange 'had three helpings of lasagne … then ate jam pudding with his hands.'

We first meet Assange and WikiLeaks editor Sarah Harrison soon after the so-called Cablegate leak in late 2010. He is trying to reach US secretary of state Hillary Clinton on the phone. He wants to 'warn' her personally about the imminent public release by another party of hundreds of thousands of confidential diplomatic documents. Insisting that Clinton herself should come to the phone, Assange suggests that he is doing them a favour: 'We don't have a problem. You have a problem,' he smirks.

An 84-minute version of 'Risk' premiered at Cannes last year, months before WikiLeaks released 70,000 hacked Democratic National Committee emails during the run-up to the 2016 US presidential election, thrusting Assange back into the limelight and leading to a substantial revision of the film. Another reason for the revision was the numerous accusations of sexual harassment and abuse levelled against Jacob Appelbaum, Assange's chief technical adviser and closest confidante, in summer 2016. The new version of 'Risk' is longer, messier and much less positive than the first cut that premiered at Cannes in 2016.

For much of the film we see Assange holed up in the Ecuadorian embassy in London, where he has lived since 2012 after seeking asylum there to avoid extradition to Sweden to face rape and sexual assault charges. Assange claims that once extradited to Sweden he would be in danger of rendition to the US where he believes he could face life imprisonment or even the death penalty for espionage. Poitras does not press him on the Swedish charges or interview anyone else about it. Instead his attitudes towards women and feelings about the case emerge through remarks he makes to colleagues and associates.

In a riveting scene with his barrister Helena Kennedy, she advises Assange to be careful about the language he uses during his extradition hearing, so as to avoid sounding like a misogynist prat. He nods as if in agreement, then adds, 'not publicly, at least,' and proceeds to make snide remarks about a 'radical feminist conspiracy' and women visiting a lesbian nightclub. Poitras does not focus on Assange's face whilst he rants about the case but instead on the appalled expression on Kennedy's face and the embarrassed reaction of another woman sitting nearby.

It is the accumulation of such small details that makes Poitras's film so quietly devastating. Later, Assange tells Poitras that it is in his Swedish accusers' best interests to drop the charges against him. His reasons are revealing: If they do not drop the case, he says, 'they will be reviled forever by a large segment of the world's population.' It is in such revelatory 'private' moments that Poitras's approach bears most fruit.

Given what WikiLeaks stands for, it is ironic that Assange demanded that such scenes be removed after seeing the first cut at Cannes. Poitras tells us in voiceover how Assange screamed at her over the phone after the Cannes premier. Yet she never shows him yelling in the film, choosing to select the more subtle moments when he flexes his muscles in the name of leadership – a collection of little instances in which women (and we the audience) are made ill at ease by him and other male colleagues. Kennedy's unease is mirrored at a seminar where Jacob Appelbaum jokes that using privacy tools online is like using a condom during sex. The women in the room shift uncomfortably, but say nothing.

When it emerged in June 2016 that Appelbaum had also been accused of abuse of power and sexual misconduct, Poitras realised that the issue of sexual intimidation could not be avoided. From her production diary, Poitras interjects: 'This is not the film I thought I was making. I thought I could ignore the contradictions. I thought that they were not part of the story. I was wrong. They are becoming the story.' The longer she worked on the film, the more concerned she became about other matters, such as the tone of WikiLeaks' Twitter feed, the timing of leaks concerning Hillary Clinton, and attitudes towards women among cyber-activists. She spent a year re-editing in light of such concerns.

In the re-cut film, Poitras weaves into her main WikiLeaks story other aspects of the wider cyber-activist world. We see Daniel Ellsberg accusing the military of gross maltreatment of Bradley (later Chelsea) Manning by imposing a massive prison sentence on him for leaking to the media. We witness Jacob Appelbaum (who featured prominently in the earlier cut of the film) speaking to an enthusiastic public audience in Cairo, denouncing censorship of Twitter in Egypt during the Arab Spring. Assange is much aggrieved in 2013 when Poitras goes off to meet an anonymous whistleblower who turns out to be Edward Snowden. WikiLeaks is not entrusted with the Snowden revelations.

The re-cut film's concluding sequence includes Assange expressing distaste for Hillary Clinton. Later, when WikiLeaks publishes documents damaging to the Democrats, he is accused of co-operating with the Russians to influence the US election. In the meantime, Appelbaum has been ejected from the non-profit making organisation where he works due to charges of sexual misconduct; he refused to be interviewed for 'Risk'.

In the revised film, in which Poitras discloses an earlier brief fling with Appelbaum, she includes footage of a 2016 hacker conference, filmed shortly after the Appelbaum scandal, where a panel of women questions the 'degree of sickness within the community that allows [sexual assault] to happen.'

In effect, the re-edited film poses the question of what we risk losing when men like Assange and Appelbaum dominate the hacktivist community. Swedish prosecutors have dropped their investigation into the allegation of rape against Assange because there is no prospect of charging him 'in the foreseeable future.' Questions regarding Russia's interference in the 2016 US election remain open. Accordingly, the ending of Poitras's film is equally, and appropriately, unresolved.

Poitras's film shows Assange in thrall to his own stardom. He is vain, arrogant and self-obsessed, with delusions of grandeur. At one point Assange says to Lady Gaga, who is conducting a hilarious interview with him about who is 'after' him and what food he likes: 'First of all, let's stop pretending for a moment that I'm a normal person. I'm obsessed with our political struggle. I'm not a normal person.'

Rather than simply damning him, Poitras weaves this contradiction – a man who demands openness and transparency from everyone but himself and his own organisation – into the very fabric of the film. Refusing tidy conclusions, 'Risk' conveys a feeling more than it presents facts. By emphasising the ongoing, open-ended nature of the stories and issues that it is following, this radical film subtly illuminates places that conventional documentaries cannot reach.