On paper Ralph Steadman is Britain’s most scabrous cartoonist, happy to cast a politician as a flow of human excrement. In person he is melancholic and slightly befuddled, a victim, you might conclude, of time, disillusionment and Celtic gloom. Yet by the end of our encounter in a London club, there has been a transformation. He has shed the latter half of his 70 years, grown a few inches and donned a white canvas hat. As his picture is taken, he cackles and fantasises about blowing up his neighbour’s tractor. Having, over 90 minutes, gently accelerated from nought to gonzo, he has turned into Hunter S. Thompson.

Thompson was the father of gonzo journalism and author of Fear and Loathing in Las Vegas. If you haven’t read him, imagine Tom Wolfe on hallucinogenics. In Feburary 2005, aged 67, he put a gun to his head and killed himself. Steadman had met him 35 years earlier, at the inspired suggestion of an obscure US sports magazine that wanted them to collaborate on an article that came to be known as The Kentucky Derby is Decadent and Depraved. On this assignment, Thompson, the writer, abandoned his reporter’s notebook and objectivity in favour of mint juleps and attitude; Steadman mislaid his crayons and drew instead with bile and lipsticks.

Neither looked back, but while Thompson settled for repetition, Steadman in his solo career notched up major accomplishments in graphic works on Freud, Leonardo and God. Now comes The Joke’s Over, a memoir of his friend. Thompson, who once told him: “Don’t write, Ralph; you’ll bring shame on your family”, would have been furious at its accomplishment.

I tell him that, as I read it, I wondered how much of a friend Thompson really was. Steadman says he wondered the same as he wrote. “I suddenly realised that it was no longer a friendship, that it was more a business deal.” A Thompson letter, you see, might read: “Ralph, your baroque style of psycho-gibberish is always appreciated here but what I really need is a six-month loan of £50,000 at whatever per cent rate you can handle. Keep your advice and send money. Thanks.”

Thompson ranted at Ralph for taking on lucrative projects such the Oddbins adverts, reneged on commitments to provide words for their books, demanded money and undermined his extramural projects. And yet, as they say, and yet . . .

“He was marvellous and his unhinged (if that’s the right word) writing released many things in other writers, like Will Self. If they don’t do it properly, they’re likely to lose their job; if they’re good at it, it’s a form of writing that gives far more than you usually get.” And personally? “I found the whole gesticulating, berating, monstrous storm of his personality amusing.”

Even though he must nearly have been killed by him, if not with drugs, by his driving, and if not by his driving, by his guns? “I was fatalistic, I suppose. Just one more mission. You know: ‘You have to do it, Ralph’.” He reflects. “I was thinking about my name. Had it been Trevor, it wouldn’t have sounded so good to him. RALPH was a kind of a bark: ‘Let’s do this, RALPH’.”

The call came at 3am from Thompson’s friend Joe Petro. “He said: ‘Take your phone off the hook. Hunter’s just committed suicide’. I thought, ‘Oh, he’s done what he said he was going to do’. It was a shock but I didn’t take in how great a shock until later. I’ll tell you what it’s like: as if one’s life is a land mass and a cliff face has crumbled away. I still don’t know if it was the interesting bit or just a carbuncle.”

In a sense Thompson’s death and its manner was no surprise. In a 1979 drawing Steadman had predicted it. “I did a picture of him scowling and holding the gun and pulling the trigger and it goes bang and my head comes out and my head goes bang.” But not for nothing does his memoir’s title announce an end to laughter. The bugle blast with which Steadman used to greet the dawn in his home in Kent is heard less often now. The fun has seeped away from life.

As he talks, Anna, his wife of 35 years, sits next to him listening, sceptical but loyal. I am grateful for her presence; she acts as his fact and mood-checker and completes sentences he has let dwindle away. I ask if she was concerned for her husband after Thompson died. “Yes, I was. He went to the doctor but she just gave him antidepressant pills, which he took for a couple of weeks or so.” Did he need them? “No. I think it’s all to do with his creativity. I don’t know: you go to a doctor, she sees you and makes a diagnosis in five minutes. I mean, she’s been our doctor for quite a while but she doesn’t know us that well.” “I don’t think she knows much about medicine,” Steadman jokes. Had he felt suicidal? “That’s what she came up with!” What did he tell her? “Contemplation is one thing, but actually doing it? No, I wouldn’t do that.”

Although he calls the death inevitable, he also thinks that it was prompted by George Bush’s victory over John Kerry, Thompson’s friend since the 1970s. Thompson hated Bush even more than he hated Nixon. “After his book Fear and Loathing on the Campaign Trail, people asked, ‘Wasn’t he a buffoon?’ I’d say, ‘Absolutely not’; his intelligence was driving us towards the awkward places in politics, like the Bush-Blair thing. You know he called Blair ‘a simpering little whore’? He called Lyndon Johnson ‘a cheap pimp who buggered children’. His friends said, ‘You can’t say that, Hunter. It’s not true’. He’d say, ‘I know it’s not, but I want to hear the bastard deny it’.”

Only Steadman, perhaps, hated the political class more. This was a cartoonist who cast Spiro Agnew as a turd being evacuated from Nixon’s bottom. But in the end even his imagination was defeated by Thompson’s command that he draw Bush as the “incarnation of evil”, a logically impossible request that Steadman still managed to stitch into a personal biography in which failure is over-represented.

The son of a commercial traveller and miner’s daughter, Steadman crashed out of Abergele Grammar School in North Wales at the age of 16 and took a variety of unpromising jobs, to the evident pleasure of his sneering former headmaster. During National Service he enrolled in a correspondence course in cartooning and sold his first work to a newspaper at the age of 20.

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