"Count five and tell the truth," Julie Christie's Liz tells the eponymous fantasist in Billy Liar (1963).

Billy (Tom Courtenay) lies to make his small-town life more interesting; lies to get laid; and lies, most of all, to get himself out of trouble. What clever Liz is able to recognise is that Billy has become such a habitual liar that a story leaps to his lips before the facts get a look in.

We tend to present lying as a more complicated matter than telling the truth: The former is "embroidering" or "counterfeiting", where the latter is being "straight" or "unvarnished". What Billy Liar astutely observes is that honesty can be much harder work than a spot of dissembling. It requires exposing rather than protecting yourself, and accepting an unpredictable response rather than one you've been able to plan for.

It creates vulnerability -- and it can make people look really, really bad. That's why, even in a political culture that obsesses over authenticity or the lack thereof in leaders and prospective leaders, actual honesty in public figures remains as two-edged a business as it can be in personal relationships. How honest do we really want each other to be?

The clash between personal honesty and public esteem is enacted in extraordinarily frank fashion in the new documentary Weiner, which covers the efforts of a rising star of American politics to salvage his political future amid a flurry of embarrassing revelations about his private life.

Anthony Weiner was a Democratic congressman who was known for his impassioned debating style, hot temper and power marriage to a top Hillary Clinton aide, until a social media mishap ensured that history would recall him for something else entirely. Ever accidentally sent a private email to a large number of people? Well, be thankful you haven't sent a photo of your excited crotch area to EVERYONE ON TWITTER.

For that is what Weiner did, having struck the wrong button during an exchange with what turned out to be quite a number of special online friends.

Weiner resigned from Congress, but he wasn't done. The film joins him as he makes a bid to be Mayor of New York City, which looks to be going rather well until another lady outs herself as having experienced his digital attentions.

Things fall apart. Pundits and talk show anchors have a field day. Weiner's wife, Huma Abedin, struggles to maintain grace under pressure, continuing to support his campaign even as her marriage becomes a global laughing stock and she obviously wants to boil him in oil.

It's agonising, compelling stuff -- but most fascinating of all is what it reveals about Weiner's relationship with the truth. Confronted with the new testimony going public, we see his inner Billy Liar surge to the fore, as he anxiously tries to establish the detail of the woman's story, and frets that he doesn't have "access" to her "timeline". What he barely seems to consider is that there's a true story, to which he does have access, because he was there, writing it. It's as if Weiner has forgotten that it all actually happened. His one concern is establishing a "narrative" - the least damaging one possible.

One might say, and many have, that this desperate striving after untruth is indicative of Weiner's basic moral weakness. Undoubtedly the man has a disturbing approach to marital fidelity, a dearth of impulse control, and a disregard for personal security that could be considered beyond the pale in public office. But his visible writhing away from the "unvarnished" truth also speaks to a basic fact of political life, which is that the ugly, silly facts of human life - we lie, we screw up, we have egos and rebellious libidos and imperfect relationships - acquire a doom-laden moral power when exposed to the glare of a political campaign.

Is the world going to applaud Weiner for being honest about having a fragile ego and a risk-taking sexual make-up? Not a chance - so as far as he can see, he might as well try to change the past. At one point we see a member of the public rail at him for his indiscretions. Weiner retaliates, demanding of the man, "Are YOU a perfect person?" "I'm not running for public office," his accuser says. Meanwhile, public attackers such as Donald Trump -- habitual bankrupt, liar, and remarrier -- have no hesitation in painting Weiner as the worst of the worst.

Weiner clarifies that the self-righteous clamour for "honesty" is very often no such thing: It's a clamour for purity, one that's shamelessly raised by those who are themselves defiantly impure. God forbid you fail to be honest; but God forbid again that your honesty reveals you to be less than that mythical "perfect person".

Think of poor Jimmy Carter, who told an interviewer in 1976, "I've looked on a lot of women with lust. I've committed adultery in my heart many times." Undoubtedly Carter was not only being honest, but expressing a fact common to countless voters of all political and religious persuasions. He dropped fifteen points in the polls, almost lost the election and was dogged by his quotes unto perpetuity. And that was before social media, which at the first sign of weakness doesn't so much throw the first stone as build a guillotine and commence the gleeful knitting.

UK politics is no more forgiving of the injudiciously upfront. Was Andrea Leadsom sincere when she indicated to an interviewer that she thought herself a better candidate for Prime Minister than Theresa May by virtue of motherhood? Almost certainly; but the resulting opprobrium ended her bid. Meanwhile, was that the real, unvarnished Jeremy Corbyn claiming at the Durham Miners' Gala, "There's no pressure on me. None whatsoever"? No. Of course there's pressure on him. He's trying to do an extremely hard job and being continually attacked.

But even this most defiantly unspun of politicians recognises that his base doesn't want to hear the negatives. The fact is that Leadsom was probably being more honest -- but what she said was less appealing, more revealing, and hence less politically effective.

Count five and tell the truth? It's always an option - just not necessarily a winning one.

Weiner_, directed by Josh Kreigman and Elyse Steinberg, is on selected cinema release and on iTunes now, and released on DVD on 22 August._

Hannah McGill is a writer, critic and broadcaster based in Edinburgh. She writes for Scotland on Sunday, Sight and Sound, The Independent and The Times among other outlets. From 2006 to 2010 she was the artistic director of the Edinburgh International Film Festival.