In 1985, just hours before Bruce Robinson was due to start shooting Withnail and I, he had a crisis of confidence. Drinking vodka at 2.30am to quell his nerves, he shared his terror with a producer. “I had never directed anything,” Robinson says now. “And I was speechless with fear.”

Spend time with Robinson and you’ll realize that speechlessness is not his default mode. This well-preserved septuagenarian is a generous and companionable presence but get him onto anything that matters to him and his conversation flows, dips, dives, rages and zips from one unguarded tangent to another. ‘Twas ever thus, he says. So he was glad when Withnail’s veteran co-producer, David Wimbury, ordered him to relax. “He said, ‘It doesn’t matter. The cast know how to do it or they don’t. You’re either going to get lucky or you’re not.’ And thank God we were lucky.”

Bruce Robinson, the writer and director of Withnail and I, at home in Herefordshire.

Were they ever. Released in 1987, and set at the end of 1969, Withnail and I is one of the best-loved British films of any era. The dialogue was endlessly quotable — “we’ve gone on holiday by mistake”; “we want the finest wines available to humanity!” — and it made an instant star of Richard E Grant as the grandstanding, soaringly sozzled Withnail. Robinson’s semi-autobiographical depiction of two unemployed actors who leave their grotty London flat for a Lake District cottage owned by Withnail’s gay — and predatory — Uncle Monty became the very acme of a cult hit. Now it had made its debut as a stage play at the Birmingham Rep. Can Robinson get lucky a second time?

“What’s so weird,” he says, “and it’s by accident rather than design, is that it doesn’t age. How many films from the 1980s have still got their noggin above the waves like that? Not many.” Since Withnail, he has directed three other films and written a total of 42 screenplays — “most of which haven’t been made and half of which are bloody awful” — but his fame still rests on his greatest creation.

“It’s the only thing I am known for, really,” he says, sipping zero-alcohol lager in a deep armchair in his book-lined living room on the Welsh borders. He and his artist wife, Sophie Windham, spent the first ten years of their marriage in Los Angeles but have lived in this sprawling farmhouse since 1994, along with their children, Lily and Willoughby, who are now grown up. On the wall there is a giant picture of Keith Richards painted by his friend Johnny Depp, who starred in The Rum Diary, Robinson’s 2011 adaptation of the Hunter S Thompson novel. Above the baby grand is a lifelike portrait (not by Depp) of Robinson himself, reclining in the chair in which he sits now.

According to its director, Sean Foley, Robinson’s stage version of Withnail is “97.9 percent” faithful to the original. The dynamite dialogue is pretty much all there, although mostly kept indoors. Gone is the scene in which Withnail and Marwood (the surname of the “I” character) are threatened by a bull in a field, and in its place is a sword fight that was shot for the film but didn’t make the final edit.

“What’s so weird, and it’s by accident rather than design, is that it doesn’t age.”

“I wrote a few new lines,” Robinson says. “But I spoke to Richard E about it. He said, ‘People will want it to be Withnail and I. They don’t want to hear new dialogue or have Danny the drug dealer be a woman.’” Robinson has chosen to stay away from rehearsals, where the actors Robert Sheehan (Withnail), Adonis Siddique (Marwood) and Malcolm Sinclair (Uncle Monty) are playing the leads. “I didn’t want to be a nosy neighbor. Half of me wants to get in my car and go to Birmingham but I’d only f*** it up.” Never any question of him directing it, then? “No, I’ve already done it how I wanted it.”

He had spent years turning down offers to put it onstage. Finally, the passage of time and the persistence of the producer, George Waud, changed his mind. “I’ve got far enough away from it to accept that it will be different.” He knows that I have seen a rehearsal and asks my opinion, just between us. I tell him that, from the half-hour I saw, it looks very promising. As Monty, Sinclair cuts a notably more refined and vulnerable figure than Richard Griffiths did in the film, should anyone be worried that the character is a stereotype of a predatory gay man (something Robinson and Sinclair, who is gay, deny).

Robert Sheehan, left, as Withnail, and Adonis Siddique, as Marwood, rehearse at the Birmingham Rep.

Robinson looks glad, although he has a couple of unspecified worries that he is waiting to set his mind at rest about when he finally watches a preview. “It’s difficult letting something go. And if there is one thing in my life that I feel closest to, it’s Withnail.”

Popular legend has it that Robinson is the reserved Marwood and that his friend and flatmate Vivian MacKerrell, a well-born, unsuccessful and alcoholic actor who died of throat cancer in 1995, is the rapier-witted Withnail. Robinson says the truth is more complicated: the two characters are a mix of him and his flatmates from their stint living in Camden Town, north London. And it was another flatmate, the actor Michael Feast, with whom he went on a trip to a country cottage.

“I mean, Withnail is more me than anybody. Back then I was very much like I am now: I’m yabbling and yacking on and I can’t help it.” He dedicated the published screenplay to MacKerrell, though, because he knew he was dying.

Robinson is more equable than Withnail and was more focused in his substance abuse: “I grew up in this environment of alcohol and rage. So did Richard E, which is why he was so perfect for the part.” Like a more reasoned Withnail, Robinson does like going off on one. He claims not to be party-political but soon vents spleen against the government, the Metropolitan Police, Liz Truss, the hounding of Angela Rayner and endemic greed.

As you’d expect from a writer who is still very much available for hire — he has just extricated himself from a bad Hollywood experience he can tell me about only off the record — he also has a pragmatic streak. “You’re a part of the whole,” he says. “It’s the reason you get paid a lot of money whether the thing is made or not.” And yet there are some things Robinson really can’t let go. Behind us on his shelves is a DVD of Oppenheimer, which he cannot bring himself to watch because he is still incensed about the fate of his screenplay for the 1989 film Fat Man and Little Boy.

He wrote it for Roland Joffé, the director with whom he had his first screenwriting success, with the 1984 film The Killing Fields, about the Khmer Rouge in Cambodia. After Joffé sacked him, the production asked for access to Robinson’s research. “This is after they’ve completely f***ed my screenplay. And I sent them a fax saying, ‘You stole my car, don’t expect me to buy the f***ing petrol.’” A zinger worthy of Withnail. “That was the end of my relationship with Roland.”

“They don’t want to hear new dialogue or have Danny the drug dealer be a woman.”

Yet Robinson thrives on a sense of injustice. “As a writer I am always looking for the thing to be angry about,” he says. What drove the writing of They All Love Jack, his 860-page 2015 book about Jack the Ripper, was his conviction that there had been an establishment cover-up to protect the true culprit, who Robinson insists was Michael Maybrick, a Freemason. A forthcoming short-story collection, titled Several Stories of Different Lengths, begins with one called The Birth of Boris. “I think they’re very funny but they’re all viciously political and angry as f***ery. I’m 78 but I still get bloody annoyed by things. I’m on fire all the time.”

What score was he settling with the sad yet endlessly witty Withnail and I? His difficult childhood in Broadstairs, he says. “All of that rage of my childhood is implicit in Withnail’s rage.” He was four, he believes, when his mother told her husband that she’d had an affair with an American serviceman while he was serving with the RAF during the war. Robinson was not his son. “He punished her by hurting me. I had an extremely violent stepfather who beat me and, genuinely, bashed me about a lot.” He failed his 11-plus and went to “a bloody awful sec mod” which happened to have a good drama society. He took leads in school plays before going on to study at the Central School of Speech and Drama in London.

From left: producer David Puttnam, director Roland Joffé, and Robinson, the screenwriter, on the set of The Killing Fields, 1984.

But despite a role in Franco Zeffirelli’s Romeo and Juliet (the predatory director informed the character of Uncle Monty), films for Ken Russell and François Truffaut, and a role alongside Marianne Faithfull at the Royal Court in 1968, Robinson’s acting career faded. Meanwhile, Lesley-Anne Down, his girlfriend in the Seventies, was becoming a star. “I never enjoyed acting, not in front of 500 people anyway. I’m a great actor on my own. I think of a character and I sit here talking to myself.”

A penniless Robinson first wrote Withnail and I as a novel in the winter of 1970-71. “I had the sense that, aged 23, I had completely failed. And instead of weeping, I laughed and wrote the book.” It wasn’t published but was passed around among a few people, and led to a screenwriting contract with the producer David Puttnam — which included, eventually, The Killing Fields. “If The Killing Fields had flopped, I wouldn’t be sitting here now,” Robinson says. “It’s what made Withnail possible.”

He still loves his less celebrated second film, How to Get Ahead in Advertising (1989), which also starred Grant: “It’s just me ranting, really.” After “a gruesome time” on his next film, the thriller Jennifer 8 (1992), he gave up directing, only going back to it after Depp asked him to adapt The Rum Diary. “Most people don’t really get it but Johnny likes it and I like it. It’s funny.”

He would love to direct one last film, from his autobiographical novel of childhood, The Peculiar Memories of Thomas Penman (1997). He has written the screenplay and is looking for backing. “But everything is so hard. If you’ve got a horse in a film, it’s got to have wings. Everything’s got to have a machine gun up its arse. If I walked through the door with Withnail and I now, forget it.”

The Penman book has already had a second life, leading Robinson’s biological family to him a decade ago. His father, Carl Casriel, a New York lawyer, died without telling his three American children about his wartime romance. However, when they discovered letters from a Mabel Robinson that mentioned Bruce, her son by Carl, they started googling and made contact. Robinson found out who his father was. “It was too late, though. We’ve all tried to keep in touch but … I didn’t see a photograph of my real father until I was 68. It’s a life gone now.”

Evaporated opportunities are a speciality of Robinson’s, who remembers his sense that the game was up for the Sixties when the Beatles began to split and, yes, when he saw a hippy wig at his local Woolworths. After almost 40 years of talking about it, he understands a part of why Withnail keeps touching a nerve. “Withnail doesn’t want to grow up, Marwood does — that’s the whole thing. And yet everyone, including me, would love to be as sharp as Withnail.”

So Robinson was lucky, yes, but he knows that his anger and wit have proved an invaluable pairing. “If you’re on that bandwidth, it’s bloody funny, Withnail. And that’s the thing I kept saying about the play: for God’s sake, if I can give you one note, the characters don’t know they’re funny. The moment you play it for laughs, you’re lost.”

Withnail and I is on at the Birmingham Rep until May 25

Dominic Maxwell is a commissioning editor and writer for Culture & Books at The Times of London