The majority of people, I suggest to Ralph Steadman, when required to recall the most excruciating embarrassment they have suffered in their life, need to stop and reflect for a moment or two. I don't have that problem.

"Oh, really?" the artist asks. "Are you short of terrible memories to choose from?"

"No," I tell him. "I've got plenty. It's just that one stands out rather prominently. And it was your fault."

Download the new Independent Premium app Sharing the full story, not just the headlines

"I very much doubt that," Steadman replies, shiftily.

The incident in question, as I remind him, occurred at the memorial dinner for the writer Hunter S Thompson, in the dining-room of the Jerome Hotel in Aspen, Colorado, a few weeks after the writer shot himself in February 2005: an event attended by 200 or so including, as well as family members, Johnny Depp, Benicio Del Toro, John Cusack, Josh Hartnett, Sean Penn and Jack Nicholson. After dinner, several speakers, Steadman included, took it in turn to mount a small stage and address the gathering.

"Do you remember," I ask him, "what you did, just before you got up to speak?"

"I don't."

"You leant over to me and whispered: 'I am going to say: Are there any other rich bastards in this room?' At which point you insisted I should raise my arm, 'because none of the rest of them will'. Then you said you were going to point at me and say, 'Well, Hunter Thompson is dead, and we are both the poorer for it now.'"

"Are you sure?" the artist asks.

"Yes. The trouble was that, when you asked that question, and I raised my arm, you forgot all about your punchline. You started banging on about something else – horses, I think it was. I was sitting about a yard from Bill Murray. I can still see him staring at me, in my $19 Gap shirt. I don't think you've ever really explained what came over you."

"Had I had a drink?"

"Possibly."

"I do remember that now, yes. It wasn't supposed to happen like that. I got distracted by something. As I remember, I looked over at Jack Nicholson and noticed that he had this plate of buns. With pink frosted icing, and little silver decorations on them…" Steadman pauses. "The thing is, you have to let these things go, like I do. I had totally lost the plot that night. You of all people should have noticed that. I occasionally get led astray by my own thoughts."

We're talking in the living-room at his home in a village outside Maidstone, Kent: a large property whose extensive grounds lend it the appearance of a small stately home. He lives here with his second wife, Anna; a section of what Hunter Thompson referred to as "Steadman's Castle" is occupied by their daughter, Sadie, with her husband and their two young children. We first met here in 1996, prior to our first collaboration in journalism, since which time I have worked and travelled with the painter on many occasions; excursions that included several trips to Woody Creek, to visit the author of Fear and Loathing in Las Vegas and, since Thompson's suicide, to spend time with his widow Anita and the writer's son Juan, and his family. The artist has become a close friend and unofficial godparent to my children, but I have never recorded a formal interview with him before, and the experience of doing so feels very peculiar.

"I know," Steadman says, adding, with mock desperation in his voice, "especially because you have had the good fortune to catch me on possibly the lowest day in my entire life."

In fact, 2013 promises to be an outstanding year for Steadman – not that fame or money have ever been prime motivators in his life. Bloomsbury has just published his new work, Extinct Boids. His greatest original art collection for several years, it's a project which began when the film director Ceri Levy asked for one piece of art for an exhibition, Ghosts of Gone Birds. After which, Steadman recalls, the birds just kept on coming. As well as genuine lost species such as the dodo and the great auk, he has drawn the Nasty Tern, the Gob Swallow and the Needless Smut. The presence of the Jailbird and the Wizened Twit did not deter one reviewer from remarking that Steadman "has paid tribute to some of the most beautiful creatures ever to have lived".

A new biographical film of Steadman, For No Good Reason, directed by Charlie Paul, which had its premier last autumn at the London Film Festival, will be released on both sides of the Atlantic later this year. An intimate and faithful portrait of the artist, it begins with him explaining his painting technique, as he works on a single image, to Johnny Depp, in the painter's Kent studio. (The two first met at Owl Farm, Hunter Thompson's home in Woody Creek, a decade before Depp played the writer in Terry Gilliam's 1998 film of Fear and Loathing in Las Vegas.) Fifteen years in the making, it's a brilliant piece of work by Paul, who had the courage to abandon the current trend for interspersing his narrative with innumerable testimonials from professional talking heads, as though the main subject required constant validation. The conversation between Depp and Steadman, which ties the film together, offers exactly the same kind of fascinating insight that anyone who has sat at the artist's shoulder as he works will recognise.

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

("You know… if you weren't here," Steadman tells the actor at one point, "I'd probably be off for a sleep now.")

His distinctive labels for Jim Caruso's Flying Dog Brewery have helped turn beers such as the challenging Raging Bitch into globally recognised, thriving brands.

Where his book project is concerned, Steadman tells me, "I started by doing a dodo for the cover. Then by the end of the first week I'd done another 10 birds. I thought, 'Oh. This is a bit more interesting than drawing Nick Clegg.'"

"It's funny you say that, because a lot of your animal images – I'm particularly fond of a painting you did for Flying Dog, which shows a flea called Old Scratch – are of loathsome and repulsive creatures whose expression suggests that they consider themselves rather handsome and beguiling. In that sense, you could see a connection with your view of our elected leaders."

(Steadman rarely caricatures politicians these days, having noticed that some of them enjoy it.)

"Politicians," he replies, "do share that ability to be hideous, utterly obscene and at the same time massively proud of themselves; they are visibly puffed up with pride and self-importance. They remind me of show dogs."

Whether you judge him on his classic artwork in books such as Alice in Wonderland, Treasure Island and Animal Farm, or the grotesque visions that inhabit his original paintings, Steadman is a phenomenon of the kind that barely exists any more: a highly successful artist who has scorned the account-executive instincts of the arbiters of taste in the British art market. His reputation as a serious painter has suffered grievously as a result of his background as a cartoonist and illustrator.

"He is a supreme talent," says Bruce Robinson, friend and director of Withnail & I and The Rum Diary. "I feel so fortunate to know him, because at his best, which is most of the time, he has the power of Goya. I don't say that lightly. He really does. There is no living artist in his league that I know of." In the foreword to Steadman's 2006 memoir The Joke's Over, Kurt Vonnegut described him as: "The most gifted and effective existential artist of my time."

Meeting most artists, you can generally detect something of their personality in the work. You might take the example of Hunter S Thompson who, for all his wit, sensitivity and imagination, was indistinguishable in his darker moods from the kind of violent, unforgiving monster whose voice resonates from some of his pages. On two occasions in Aspen I have seen him reduce Steadman to tears, and that was just while the American was still alive.

Ralph Steadman himself is a different matter. Adjectives such as compassionate, benevolent and selfless have become as chronically devalued as that other term often applied to him: genius. And yet it is hard to comprehend how this modest person who has (I write the following lines in the anticipation of ridicule but anybody who knows him will have seen it to be true) an intuitive connection with animals and young children, who appear drawn to him, as a kindred innocent spirit – could produce work of such fury and violence. Not so long ago I was talking to him about a leading European thinker currently living in London. The next day Steadman sent me, unprompted, a painting of the man that was so graphically disgusting I can't think of any mainstream magazine that would print it. (It was Alain de Botton.)

On the face of it, it's hard to think of any creative artist whose apparent character is further removed from their output. The kindest, elderly female librarian, furtively writing detective stories about the most barbaric of mass murderers, could not begin to challenge this genial, engaging man of 76.

"In many ways," Johnny Depp tells me, "I look upon Ralph as a kind of fucking miracle. It is just a gas to go down to see him in Kent and spend time with him and his family. I was there last year; it is an incredible privilege; and he really is just so gentle and so… nice. And yet at the same time he is… you know… a psychopath."

Ralph Idris Steadman was born in Wallasey, Cheshire, and grew up in Abergele, North Wales, from the age of five. I have always had the sense, I tell the painter, that he must have been raised with a strong notion of the importance of moral justice.

"Well, I was brought up to be an honest person by my mother and father. When I was growing up, honesty was the prime foundation in anyone's life. That idea was ingrained in me as a little lad."

"And that relates to your art, doesn't it? You're taught to believe that people should not boast, or exploit others, or maim, or torture. Then when you do have to confront subjects such as Vietnam, Iraq, or Guantanamo Bay," I suggest, "your reaction is almost like road rage."

"I'd discovered at an early age how dreadful certain people can be. We had a lovely headmaster when I first went to Abergele Grammar, DB Jones. Then this hideous monster took over; this total shit who carried a cane. From then on I loathed that kind of authority figure. Because I had been brought up to believe that there was good in the world."

Steadman dropped out of an engineering apprenticeship at aircraft manufacturer De Havilland after less than a year "because I couldn't stand life in the factory" and went to work at Woolworths in Colwyn Bay. He began drawing seriously while he was completing his National Service.

"Then I enrolled in a correspondence course taught by Percy V Bradshaw, called You Too Can Learn to Draw and Earn £££s."

His real mentor was the art teacher Leslie Richardson, who was teaching at East Ham Technical College when first approached by the aspiring cartoonist. That was 52 years ago and the two men remain close.

"Ralph told me about the correspondence course," Richardson recalls. "I loathed them. I thought they were a rip-off, especially for working-class people. I also didn't particularly want to teach cartoonists," he adds. "If I took a cartoonist, I wanted them to aim at Goya, at Daumier and at Toulouse-Lautrec."

Richardson also taught Gerald Scarfe, who was introduced to him by Steadman. Though the two artists are often confused, even today, they were, according to Richardson, "very different. Scarfe's drawing was clinical in character, suited to caricature. Ralph's work was always much more emotional. It was fuller and far more expressive."

Perhaps Steadman's greatest achievement has been to remain more faithful to the original subversive spirit of his art than to the lure of the corporate dollar.

Being taught by Richardson, Steadman says, "You'd take in photography, literature, philosophy and the history of art. He gave you an education."

When Ralph met Anna, on New Year's Eve 1969, he was still living in west London with his first wife Sheila. They had four children. The dawn of the 1970s would herald two new marriages: one domestic, contented and ongoing; the other artistic, turbulent and somewhat less predictable.

There may be readers still unfamiliar with the name of Hunter S Thompson. In his 1971 masterpiece, Fear and Loathing in Las Vegas, he took the Wodehousian bachelor's blithe and adventurous attitude to alcohol and extended it to LSD and munitions. He had a life-long attraction to firearms and tear gas, and a history of igniting marine flares in situations of no obvious nautical emergency: he detonated one in a Manhattan pizzeria while he was having lunch with Tom Wolfe. Once he had established his international reputation, Hunter Thompson brought the hubris of a delinquent rock guitarist to the normally sedate world of American letters and is usually recognised, along with Steadman, as the principal exponent of so-called gonzo journalism.

The two men first collided in 1970 when they were sent by Warren Hinckle III, editor of Scanlan's Monthly magazine to cover the Kentucky Derby. They subsequently travelled several times together, notably on their infamous trip to the America's Cup (also in 1970, and also for Scanlan's) and to Kinshasa, Zaire, to cover the George Foreman-Muhammad Ali fight for Jann Wenner's Rolling Stone in 1974.

The drawings for Thompson's drug-fuelled novella Fear and Loathing in Las Vegas were done in England. Steadman had never set foot in Nevada. In most accounts of their troubled yet fruitful relationship, the British artist is portrayed as very much the junior partner: a "mere" illustrator; an accessory.

Their relationship, to borrow a familiar image, was analogous to that of an organ-grinder and his monkey, with each believing the other to be the monkey. If you had to choose one song to describe their curious rapport, you could do worse than "The Hunter Gets Captured by the Game" by Smokey Robinson.

"I have no doubt," I tell Steadman, "that you would have emerged as a major artist even if you'd never worked with Thompson."

There is a moment during a filmed encounter I had with Thompson in Aspen in 1996 (an exchange included in the film For No Good Reason) where Steadman casually informs Thompson that Fear and Loathing might not have received the attention it did, had it not been for his artwork: a notion greeted with little enthusiasm by the man from Louisville.

"I wouldn't have made it in the same way in America, that's for certain," Steadman replies. "I felt that my work lacked something. It lacked bite; it lacked rawness; an edge."

If you worked at your best when mentally ravaged to the point of collapse, Steadman would discover, then Hunter Thompson's was a useful number to have in your contacts book. When the pair were sent to report on the America's Cup race at Newport, Rhode Island, the artist began to feel seasick aboard their yacht, and asked if he could have one of the tablets of psilocybin (a hallucinogen similar in effect to LSD) that Thompson had been swallowing "like Smarties". An hour later, Percy V Bradshaw's former pupil was in a rowing boat with an aerosol in his hand, seeing red dogs in the ocean, urging Thompson, whose shoes had gone overboard, to row faster so that he could spray the words "Fuck the Pope" on the hull of the Australian challenger, a 12m racing yacht named Gretel II.

Challenged by a police launch, Thompson detonated his stash of naval flares, setting fire to several boats in the harbour, and the two journalists escaped.

The following day, Steadman landed at New York's LaGuardia airport, where he entered the baggage hall still hallucinating, with no shirt, shoes or socks. ("I told him it was common for people to wander around New York barefoot," Thompson wrote later. "How would he know? He was British. I told him the really fastidious ones wore black socks. Maybe he didn't believe me, but by then I had his shoes on my feet.")

"Very often in these stories," I suggest to Steadman, "you emerge as reckless, headstrong yet easily led: the bully's dream. And Thompson, let's face it, could be a bully."

"I suppose he could, yes. He could be very gentle, too. But he definitely liked to see what I would do in certain circumstances. The way I've always looked at the whole experience is that all he had to do was to point me in a direction. And I would have to go that way; all the way."

"Touchy as he was about anybody mentioning it when he was alive, I think we can now safely say that, on any given day at Owl Farm, Hunter was consuming industrial quantities of cocaine, cannabis [and, by his own account, LSD]. He was the one who gave you a hallucinogen. Did that affect your subsequent vision as an artist?"

"Oh, it did, yes, definitely at the time. People forget that I was the one that suggested we row out and spray 'Fuck the Pope' on the hull of the boat. That was the psilocybin talking. They were Hunter's paint canisters, but he was actually quite shaken by that idea."

"With some reason."

"Yes. I could still be in prison. But that was the one drug experience I had in my whole life. I don't take cocaine. I don't smoke cannabis, though many people assume that I do. I smoke hand-rolled cigarettes but I have never inhaled. I do enjoy a glass of white wine."

I tell Steadman that I remember what he said as we were pulling out of the drive of Owl Farm the last time he saw Hunter alive, in the autumn of 2004, namely: "I don't think we'll ever meet him again."

"I remember that very well," he says. "It was an instinctive feeling and it overwhelmed me. I just didn't think it would happen quite so soon."

"And yet when we've been back in America since he died, I've noticed how, over there, even in the street, you seem to attract much more…"

Derision?

"Attention."

"Well [in Britain] it can sometimes be a case of 'Ralph Who?' these days. But I rather like that."

(Thompson's fearlessness proved contagious and remains a central part of his legacy to Steadman. Nobody who saw it will ever forget the painter's vocal performance at the London Barbican in 2005, part of Tim Robbins' evening of sea shanties, Rogue's Gallery. Steadman sang – beautifully – Thackeray's ballad "Little Boy Billee". His fellow performers included Robbins, Norma Waterson, Teddy Thompson, Shane MacGowan and Martha Wainwright.)

Looking back at the experiences we've had together – which include an exchange of opinion with John Kerry at the detonation of Hunter's ashes in 2005, a week-long retreat in Utah and a peculiar excursion with Terry Gilliam and Merlin Holland, grandson of Oscar Wilde, which ended up in a curry house in Croydon – I tell Steadman what I think has been my most memorable: simply the privilege of watching him work, whether that has been here in his Kent studio, in some isolated mountain cabin in Colorado, or on a makeshift drawing-board at the Oxford Hotel in Denver. I can remember being horrified on one early trip when Steadman spilt a glass of red wine on to the cartridge paper. Blended with a little cigarette ash, the stain became a central feature of the picture. There is a peculiar joy in observing pure instinct at work; in one of those moments, as Francis Bacon puts it, at one point in For No Good Reason, when "chance and accident takes over".

"As I said earlier," Steadman remarks, "I get led astray by my own thoughts. That's definitely true when I am working." What kind of thoughts? "Thoughts like, what if I just splattered that, or stuck that, on there. What would happen? The shock value of something. Like you've just drawn a vicar, and instead of his face you put a breast. With a nipple."

"You make art sound like vandalism."

"Well it can be, in a way."

It's a point made none too subtly by Hunter Thompson in his letter to Steadman after one of the artist's children had broken a jeweller's window in Maidstone; a piece called "The Pro-Flogging View", published in his 1988 collection Generation of Swine.

"Dear Ralph," it begins, "I received your tragic letter about your savage, glue-sniffing son and read it while eating breakfast at 4.30am in a Waffle House on the Edge of Mobile Bay… You are reaping the whirlwind, Ralph. Where in the name of art or anything did you ever see anything that said you could draw queer pictures of the prime minister and call her worse than a denatured pig – but your own son shouldn't want to smash windows? The prime minister is a denatured pig, Ralph, and you should beat her like a gong. Draw horrible cartoons of the bitch and sell them for many dollars to Private Eye, but don't come weeping to me when your own son takes it into his head to smash a few windows. What do you think we've been doing all these years? You were getting paid to smash windows. And that is an art in itself. The trick is to get paid for it."

Ralph Steadman learnt of Thompson's death when he got a phone call from his friend Joe Petro III. Petro, himself an accomplished artist, has printed Steadman's artwork for many years.

"Joe said, 'Take your phone off the hook,'" he remembers. "'Hunter's shot himself.'" To watch Ralph Steadman over the past few years has been to observe a man recovering from the most brutal kind of bereavement. In death, as in life, Thompson had taken the artist to some very dark places. In the past couple of years, the artist seems to have found a kind of equanimity, and has done his utmost to prevent disagreements over his late friend's estate degenerating into the kind of ugly feuding that have afflicted other probate cases, notably that of Jack Kerouac.

"When I spoke to Ralph recently," Johnny Depp tells me, "he seemed shocked when I told him how much Hunter adored him. But he did. Hunter fucking worshipped Ralph. Of course he was never going to let him know that. Hunter loved him and he thought his work was – as it is – absolute genius."

It's curious, I suggest to Steadman, that it can take many decades for subsequent generations of critics to determine what is really great art. Writing such as "The Pro-Flogging View", or Thompson's majestically vitriolic Rolling Stone obituary of Richard Nixon have already assured his place in the history of great satirists. In the domain of pure fiction – the area in which he most craved success, with tales such as The Rum Diary – he may be less fortunate.

Where his own career is concerned, Ralph Steadman is fond of remarking, he tried, as a cartoonist, to change the world, "and yet failed". As a serious artist, I suggest, posterity is likely to be infinitely more generous to him than the current generation of critics, some of whom have been snobbish to the point of insult about his newspaper origins.

"Well of course I have already passed into history," Steadman says. "I'm finished. I'm done for already. He gives a mischievous nod towards an edition of one of my novels on the shelf behind him. "Are you wondering if there's any hope for you?"

Such talk, he says, puts him in mind of posthumous tributes. He made this complaint to me once before, some years ago, sitting at a bar at Las Vegas. So what adjectives, I asked him, would he like to see in his obituary?

"Distasteful," he said. "Unhygienic. Truculent. Moody. Provocative towards bastards."

How about long-lived? "Oh yes," he said. "I'd like my obituary to say, 'He was very long-lived. Endlessly lived. We thought he'd never go away." A pause. "And we were right," he added. "He didn't."

'Extinct Boids' is published by Bloomsbury, priced £35