“F— you and your cheap drunken whining,” writes Hunter S. Thompson in his foreword to Gonzo The Art, a collection of Ralph Steadman’s drawings. Back in 1971, Steadman had illustrated Thompson’s Fear and Loathing in Las Vegas: A Savage Journey into the Heart of the American Dream, that seminal text of Gonzo journalism, and the two teamed the following year for Fear and Loathing on the Campaign Trail ’72. But by 1998, when asked to write the Gonzo foreword, he had soured on Steadman, just a tad.

Given to outrageous invective on his most genial days — for that was his art and his madness — Thompson calls Steadman “a living, dangerous holograph of Dorian Gray” and soars into a more incriminating analogy: “You are now worse than Hitler in my mind. … Adolph was also an artist [but] he took the high road and you took the low road.” Escalating the excoriation, he warns his old colleague to “Never forget that you were a thief, once, a whore and a desperate brute of the streets. You forced your children into crime and sick burglaries, just to support your foul habits. … You are doomed and I can’t help you. … Soon you will have your moment in the Great Hall, face-to-face with the Lords of Karma. Good luck, buster. You’ll need it.”

Thompson faced the dark Lords in 2005, when at 67 he blew his brains out, leaving a note that ended, “Relax — This won’t hurt.” Steadman, 77, is still around, still refining his ferociously satiric style. Using his pen nib as a predator’s talon, he created works that suggest a modern Hogarth, a grosser George Grosz, a surlier Edward Searle, a Jackson Pollock in the graffiti era or an angry child defacing blotter paper. Ceaselessly productive, Steadman has published three books in the past three years: his Book of Dogs, Book of Cats and the illustrated memoir Proud Too Be Weirrd. In his spare time he sat for a full-length movie portrait, Charlie Paul’s 2012 doc For No Good Reason, which opens in the U.S. this weekend.

(SEE: A Gallery from The Ralph Steadman Book of Dogs)

His interlocutor is Johnny Depp, a friend of Thompson who also starred in Terry Gilliam’s movie of Fear and Loathing in Las Vegas and Bruce Robinson’s film of Thompson’s The Rum Diary. In 1998, Depp and Thompson visited the TIME offices and raised some merry hell (or so I’m told; I wasn’t invited). After Thompson’s death, Depp funded the funeral service: shooting the writer’s ashes from a cannon to the accompaniment of “Mr. Tambourine Man” (the Bob Dylan song to which the Las Vegas book was partly dedicated). Among the mourner-celebrants were Jack Nicholson, Sean Penn, Bill Murray, Charlie Rose and Ralph Steadman.

Depp’s appearance in the doc, however appreciated, doesn’t bring much but the patronage of a famous, friendly dude. Nor is Paul quite up to the challenge of synopsizing and illuminating an artist’s long career. As if to prove this is a coffee-house movie and not a coffee-table book, the director uses split screens, animation and rapid montage. But the salient, liveliest parts of For No Good Reason — the title comes from Thompson’s reply when Steadman once asked him, “Why are we doing this?” — are to be found in the artist’s display of his work and recollections of the eccentrics he met.

(READ: Corliss on Johnny Depp in the movie of Hunter S. Thompson’s The Rum Diary)

For an artist who says he needed to draw “as an weapon, almost,” and whose black-and-white cartoons spill bile like those ink blots, Steadman is an amicable gent, as befits the author of many children’s books, the Gardening correspondent for Rolling Stone and the choir boy he was back in Abergele, Wales. Seeing an art-school ad in a magazine at 16, Steadman was soon contributing to Peter Cook’s satirical magazine Private Eye, where he honed the essence of his drawings: “to distort and yet maintain the likeness.” He came to New York City in 1970 — “the gray decade,” he recalls, “all these gray people staggering toward you… almost a museum of misery and deprivation.”

While in America he got an assignment to cover the Kentucky Derby for Scanlans magazine; the writer would be Hunter Thompson. He quickly realized that he had “scored a bull’s eye the first time, and met the one man I needed to meet in America.” The two seemed a chronic mismatch. “To me he was weird,” Steadman says. “To him, I was weird.” The artist rarely took drugs or alcohol; the writer never stopped. Director Brian De Palma says of Steadman, “I’ve never met a warmer, generous… He is not his paintings!” Yet Rolling Stone’s Jann Wenner, who hired Steadman to illustrate Las Vegas after another artist dropped out, says that Steadman was the more daring one, Thompson the more cautious.

Steadman’s first sketch, of a Derby poobah’s wife, so affronted the woman that Thompson figured they were screwed. He gave up on the story, simply sending his notes to Scanlans, which published them under the title “The Kentucky Derby is Decadent and Depraved” and carrying the byline “Written under duress by Hunter S. Thomson, Sketched with eyebrow pencil and lipstick by Ralph Steadman.” The artist had seen the light: that he and his partner could turn journalism into cynical, weirrd comedy with a touch of autobiography. “The very face we were looking for was us,” he says, “back in the mirror.”

(SEE: Ralph Steadman’s illustrated blog and biography)

Working with Thompson or on his own, Steadman saw himself as a social reformer. “Cartooning meant more to me than doing funny pictures,” he says. “It meant changing something for the better… to change the world,” he says of the ’70s. “Now was the moment.” One drawing, shortly after news of the My Lai massacre and the Napalm poisonings, asked “How’re ya gonna crucify a child in Vietnam without any arms?” In his strongest political work, a strain of activist optimism peeks through the misanthropy, as if Steadman thought he could destroy Nixon and Ehrlichman by painting them as monsters.

He crossed paths, but not swords, with William S. Burroughs, the junkie poet-novelist, whom he compares with Thompson by noting, “They were perverse in many ways, and yet incredibly honest.” One of his saddest images is his dream-nightmare of the ailing Thompson in a retirement home: “There was this old crone crawling across the floor, and he knew that she was going to fondle his balls.”

Steadman stays youthful by seeing every painting as a new beginning. “I go out of my way to make something that is as unexpected to me as it is to anyone else,” he tells Depp. “If I knew what was going to happen before I started, what would be the point in doing it?” We might wish that Charlie Paul had surprised himself and his audience more with this documentary. For No Good Reason is no great shakes as a movie, but it will have value if it coaxes viewers into learning more about a remorselessly gifted artist who lived longer than Dorian Gray and who, on this film’s evidence, is a lot nicer than Hitler.

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