Gonzo: The Life and Work of Dr Hunter S Thompson, Alex Gibney, 119 mins, 15

A biopic of the trailblazing Hunter S Thompson shows ugliness and folly behind the bravado
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There's a brief archive clip in Alex Gibney's documentary Gonzo: The Life and Work of Dr Hunter S Thompson in which his hero ushers a visiting Keith Richards into his den. You can imagine what thoughts are flickering behind the old Stone's mummified features: "Wow, maaan... This is where it all really happens. Forget guitars – sitting at a typewriter, that's the real rock'n'roll."

The dean of counter-culture journalism, Thompson probably did as much as the Rolling Stones to enshrine the idea of the rock'n'roll lifestyle. It came from his books: above all, his 1971 memoir of an apocalyptically narcotised road quest for the American dream, Fear and Loathing in Las Vegas. Gonzo includes a passage of Thompson's prose recalling the thrill of night-time motorbike rides on California's coast roads, marvelling about "the Edge". The tragedy of his life is that Thompson finally lost his own edge – or so dissipated himself that he simply forgot how to write, before shooting himself dead in 2005. Generations of hopefuls – inspired by the likes of Thompson's beloved Scott Fitzgerald – laboured under the delusion that you couldn't truly be an American Writer unless you were permanently drunk. Thompson raised the ante, adding that you'd damn well also better be coked up, tripping on acid and brandishing a pistol in your non-typing hand.

Named after the strain of take-no-prisoners, delirium-chasing journalism cultivated by Thompson, in cahoots with British cartoonist Ralph Steadman, Gonzo gives us plenty of the writer as wild man. We get stories of Thompson's gargantuan boozing and pill-popping capacities; endless photos of him posing with firearms; tales of him hunting boar with a sub-machine gun. Gibney also stages an awkward re-enactment of some of the larks that went into Las Vegas, and some clips of Terry Gilliam's adaptation of the book, which remind you what a futile venture that film was. There's also plenty of Thompson's ugliness and folly on show. His first wife, Sandy Thompson, recalls her shock at hearing about his affairs – less of a shock to the viewer after all the photos of him lounging with naked (and usually cigar-smoking) women.

But the tomfoolery and horror come second to the theme that Gibney really values: Thompson's status as political journalist and shit-stirring radical. Gonzo's most revealing section covers Thompson's noble but failed campaign to be elected sheriff in Aspen, standing on a Freak Power ticket: a campaign for which he shaved his head so that he could refer to the right-wing incumbent as "my long-haired opponent".

Gibney – who made the documentaries Taxi to the Dark Side and Enron: The Smartest Guys in the Room – shows us the impassioned Thompson who fervently believed in America and the utopian changes promised by Sixties counter-culture, and who was bitterly, even fatally, disappointed by that dream's failure. Commissioned by Rolling Stone to write what became Fear and Loathing on the Campaign Trail '72, Thompson pinned his hopes to noble but doomed Democratic candidate George McGovern. He consequently barracked his man's Democrat rivals Hubert Humphrey and Ed Muskie, putting it about, in an inspired libel, that the latter was addled by a mysterious drug administered by Brazilian doctors. Questioned on TV about the allegation that Muskie was addicted, Thompson coolly replies, "I never said he was – I said there was a rumour in Milwaukee that he was ... I started the rumour."

Then the rot set in. He was engulfed by his own legend and his partying: he forgot whether he was supposed to be Thompson or his crazed alter ego "Raoul Duke". Gibney gives us plenty of footage of the Duke self, cantankerous and increasingly bloated, but also photos and clips of the earlier Thompson – a pensive, reticent man, sucking on his trademark cigarette holder with aristocratic delicacy. It becomes clear that the drugs that really got Thompson high were prose and vitriol. His choice passages, silkily read by Johnny Depp, are a venomous joy, especially when directed at Richard Nixon: "so crooked that he needed servants to help him screw on his pants every morning".

Gibney has mustered a wonderfully heteroclite bunch of character witnesses: among them Jimmy Carter, former Nixon speechwriter Pat Buchanan, and Hell's Angels veteran Sonny Barger, now a wizened gnome chuckling through his tracheotomy about the beating Thompson received from his boys. Also present is Tom Wolfe, and it's mighty rich to hear him talk about the dangers of being swallowed by your image: the ostrich-necked, ice-cream-suited old patrician looks more than ever like a Steadman cartoon of himself.

Thompson doesn't get an easy ride. Some, says ex-wife Sandy, saw his suicide as a brave act; she thinks the opposite. "He could have a made a difference," she says. Nothing in Thompson's decline is quite as sad as the image of him listening to "Candle in the Wind" over and over again – an image sicker and more perverse than anything he wrote. Gonzo makes riveting, troubling, exuberant viewing, but it contains a thought to give pause to every journo who sees it. It's said that Thompson never met a deadline – even when provided with a prototype fax machine, which he promptly contrived to sabotage. We hacks who make a habit of hitting deadlines, however narrowly, can only watch Gonzo and wonder what the hell we've been doing wrong.