Alex Gibney is responsible for some of the most politically charged documentaries about American corruption in the past ten years. Enron: The Smartest Guys in the Room (2005) is a withering account of corporate ineptitude. Who Killed the Electric Car? (2006) reveals just how far the oil industry is prepared to go to defend its interests. And Taxi to the Dark Side, the horrific true story of a young Afghan taxi driver who was tortured to death at an American military air-base in 2002, won the Oscar for Best Documentary in 2007.

His latest film, Gonzo: The Life and Work of Dr Hunter S. Thompson, is a rude and brilliant homage to the most legendary troublemaker of them all.

Johnny Depp reads chunks of Thompson’s prose while toying with a handgun. There are insane clips from Terry Gilliam’s Fear and Loathing in Las Vegas; grisly footage of the Chicago race riots; newsreel of a young skinhead Thompson running for local Mayor; snatches of Nixon and Pat Buchanan in their evil pomp; arguments with the brutish Hell’s Angels; the confessions of former wives; and hair-raising anecdotes from Jimmy Carter, Tom Wolfe, Ralph Steadman, and the long-suffering Editor of Rolling Stone, Jann S. Wenner.

“I think the birth of gonzo happened when the evil in me came out of the drawings,” the elderly Steadman says. Gibney slices to the drug-fuelled trip the duo made to the 1970 Kentucky Derby that resulted in the most famous non-article about a horse-race ever written. The young Steadman’s eyeballs are rolling in different directions.

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This is the kind of mischief that makes Gibney’s documentary such a nostalgic joy. “I’m not ashamed to entertain,” Gibney says.

After graduating from Yale, Gibney went to the UCLA Graduate School of Film and Television. He made an instant impact in the early 1990s by writing, directing and producing a string of award-winning TV shows such as the ten-hour series Pacific Century.

“Werner Herzog defined documentaries as either the work of accountants or the poetic truth,” Gibney says. “That’s the original joy of Hunter. He wasn’t hung up on facts but he had a matchless instinct for the truth.”

The main influence on Gibney’s career is arguably his father. Frank Gibney was the Tokyo bureau chief of Time Life magazine after the Second World War. During the Pacific conflict he was recruited by the US Navy to interrogate Japanese prisoners during the battle of Okinawa. His memories of how prisoners were treated then simply don’t square with how the US military treats them now.

Infuriated by the scandalous techniques used at Abu Ghraib, Frank was instrumental in getting his son to make Taxi to the Dark Side. The Oscar-winning film ends with an interview with Frank just before he died. “He had to take his oxygen mask off to speak,” Gibney says.

You can’t tap that kind of visceral, personal emotion lightly. Gibney’s interviews with Thompson’s close family evoke similarly pungent memories. There are loud clues about what a miserable git the world’s most famous journalist was to live with in unstintingly generous tributes by his first wife, Sondi Wright, and his son, Juan. “When I’d come home from school, my father would wake up and have breakfast,” Juan says with deadpan frankness. Gibney admits that the film could have been much more toxic about Thompson’s personal life, but it would have illuminated precisely nothing.

Gonzo is a celebration of Thompson’s legacy, and true to his word, Gibney plays fast and loose with the books, the diaries, the letters and the facts, with acres of original material. A vintage photograph of Thompson pointing a gun at a typewriter in the snow suddenly comes to life when the gun unexpectedly goes off. A crackly audio recording of Thompson and his stoned Mexican lawyer asking a bemused waitress at a seedy taco restaurant for directions to the American Dream sounds like genuine archive until you realise the sequence has been cleverly cooked from half a dozen angles.

This is the Thompson beloved by liberals and hated by giants. It’s the Thompson that Gibney has spent most of his career as a documentary journalist aspiring to be – “though very few human beings could possibly match his capacity for drugs.” The striking irony is that this is a movie Gibney never intended to make. “I didn’t actually set out to make Gonzo,” he confesses. “It was Graydon Carter, the Editor-in-Chief at Vanity Fair, and Laurie Ackerman, a famous producer in London, who dreamt up the idea. They came to me and said would I be interested in shooting a film about Hunter S. Thompson. I couldn’t see the point.

“But the idea struck a chord. When the current administration is caught red-handed issuing false press credentials to a male prostitute [Jeff Gannon] to pitch soft questions at official White House press conferences, journalists have a serious responsibility to be a bit more imaginative about how they respond. Hence Gonzo.

“Hunter understood the American character in a way very few writers did. He understood the contradictions. He was possessed with the idealism of the American Dream and the hope it offered. Yet he was full of loathing for the ‘gunfighter’ side of the national identity that the dark side of him so eagerly embraced.”

Thompson never made any secret of how he was going to finish his career. He wanted to blow his brains out while he was still on top of his game. But even his most fervent admirers agreed that his best work – Gibney’s estimate is between 1965 and 1975 – was a long way behind him by the time he put the barrel in his mouth on the February 20, 2005.

“I don’t think his suicide was anything to be admired,” Gibney says. “I’m sure he was bipolar. His fatal flaw was that he was a hopeless narcissist. Which incidentally is why he got on so marvellously well with politicians. Thompson was a great artist. But you don’t look for balance in a great artist. You look for wisdom.”

Gonzo shows at OWE2, Oct 27, 6pm and OWE1, Oct 28, 1.30pm. It goes on general release on Dec 19

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