With hindsight, Trainspotting could never have been anything less than generation-defining: that iconic "Choose Life" monologue; the Britpop sounds of Blur and Elastica; the big techno beats of Underworld and Leftfield; the impossibly hip film team behind Shallow Grave; the wildly popular cult novel by Irvine Welsh; and – always good for box office – conservative critics tutting that the movie glamorised drug culture.

“It was all a bit of a blur,” recalls Welsh, whose premillennial partying made many a contemporaneous tabloid headline. “I was very conscious of the fact that I was suddenly getting invited to all these big parties and gigs and book launches. But I had discovered only a few years earlier that I was a writer and that writing was something I wanted to do, so that kept my focus a little bit.

“I was lucky in a way, because all the parties weren’t as much fun as getting on and writing something new. You can make a career and way of life out of partying and beer, but it’s a diversion you want to get away from in the end . . . hopefully.”

Welsh's pre-Trainspotting life doesn't quite read like Jack London's method adventuring, but by golly it's eventful. Born in Leith to a waitress and a dock worker, Welsh quit school at 16 to train as an electrical engineer until, in an alcohol-related accident, he and a friend dropped a telly. He left Edinburgh for the London punk scene in 1978, where he played ("badly" by his own account) in a series of unsuccessful bands.

After several lean years of skipping out on bills and grotty bedsits, he landed a job with Hackney London Borough Council. Compensation for falling out of the top deck of a bus when it toppled over in a traffic accident allowed him to profit from London’s property boom and return to Edinburgh, where he took a job with the housing department and began writing.

A terrible employee

“I think this is what happens if you’re a writer but you haven’t realised it yet,” he says. “You’re always ramping up. But you can only see it in retrospect. I was never emotionally involved in any job I had. I was a terrible employee. I was always looking for something else. That something else was obviously writing. Before I knew that, I used to love to accumulate experience. I thought I was going to be rootless bum or a drifter. But, basically, I was trying to find something to write about.”

Thus, as the century wound down, Welsh became the enfant terrible of English literature, blazing a trail with Trainspotting (1993), a chronicle of misdeeds and misadventure among Scottish heroin addicts.

"The best ever quote I read about my writing was from a local paper from Portsmouth," says the author. "They said Trainspotting was like James Joyce meets Viz comic and together they go on a romp through hell."

That bawdy, postmodern style continued through such bestsellers as The Acid House (1994), in which a junkie and a baby swap bodies, and Filth (1998), which was narrated both by a sociopathic policeman and the tapeworm that lives inside him.

“One of the things I noticed was a tremendous gap in Scottish literature at the time,” Welsh explains. “The people I knew were getting wasted on all sorts of drugs, whether that was heroin or ecstasy, which was just beginning to come into the picture. Not so much cocaine, but speed was a staple. And Scottish literature was: ‘Two pints of heavy, please barman’.

“There was a massive disconnect. [Alexander] Trocchi had written about drugs, but he was writing from an upper-middle-class, bohemian outsider perspective. Not as somebody living on a council estate.”

Gone the distance

These days, following a stint in Dublin ("I still miss Rathmines"), Welsh has relocated to Miami. There, having swapped waking up in swimming pools beside Noel Gallagher for green tea, he leads a quiet, bookish life with Elizabeth, his wife of 12 years. But the voices of the Trainspotting quartet – Sick Boy, Renton, Spud and Begbie – have never quite gone away. The geographical (and financial) distance between Welsh and the subculture that spawned his best-known subjects has not diminished their chatter.

“The interesting thing is that you can always write about characters that are the same age as you or younger,” he says. “Because I can identify with any age I have been or situation I’ve been in. I suppose we’re such a narcissistic, individualistic culture, we don’t really want to let go of our youth. So, for me, the problems I have is writing about people that are older than me.”

He mumbles, dryly: “Though that’s less of a problem every year.”

Last year, Begbie made his latest literary appearance in The Blade Artist. Before that, a 2012 prequel, Skagboys, charted Renton and Sick Boy's initial descent into heroin addiction. And Porno, Welsh's 2002 Trainspotting sequel, forms the spine – with several important embellishments and deviations – of director Danny Boyle's long-awaited T2 Trainspotting.

"The reason for writing a Trainspotting sequel was that Sick Boy's character kept gate-crashing into what I was trying to write," Welsh says. "I never really saw it that way. I had called the character a different name. I fought it. But I had to give in: this was Sick Boy. And once you start telling Sick Boy's story, you have to start thinking about what happened to the rest of them. It was a minor part of the book, and then it became the main thrust. It was really frustrating in the beginning."

Undented psychopathy

Arriving some 21 years after the original film, T2 Trainspotting looks certain to seduce older fans with Renton's unchanged bedroom, Sick Boy's newest hare-brained scheme, Spud's ongoing attempts to get clean, and Begbie's undented psychopathy. The film stars the original ensemble cast, including Ewan McGregor, Ewen Bremner, Jonny Lee Miller and Robert Carlyle, and also reunites Boyle and Trainspotting screenwriter John Hodge for the first time since The Beach (2000).

“I’ve got a really good relationship with John,” says Welsh. “I’ve always said that I don’t want to be a curator of my own stuff. My role in all the discussions and meetings and stuff that we had is as a research guy. I’m an enabler. I don’t want to watch some attempt to put the book exactly on screen. Film is a different art form. There’s nothing to excite or surprise me in that. I’ve done the book. I don’t want to be involved too much after that. I want to see what other people can do.”

Happily, the new film has, indeed, excited and surprised Welsh. It is, he says, a film about everything that has happened in between. That, he notes, is a lot of ground to cover.

“We can’t pay wages anymore because the economy had flatlined. So everyone lives on credit and credit becomes debt when you can’t pay your way out of it. What we believed about capitalism and industrialism is gone. How we lived is gone. So there is this great age of uncertainty.”

In 1996, Trainspotting was buoyed by its youthful Britpop exuberance. As Welsh sagely observes, with the exception of south London's grime scene, Britain hasn't produced a uniquely indigenous culture "since acid house and football hooliganism".

A lot to write about

Still, the 59-year-old author is hopeful for the future.

"I'm disappointed in Brexit; I'm disappointed about Trump," he says. "But it gives you much more to write about. In that respect, these are exciting times. The most interesting, resonant novels of our time – Fight Club and American Psycho – appeared in the last millennium. They are books about a time of transition. A bit like Trainspotting, I suppose.

"Fight Club is about the first generation of working Americans who are poor. Psycho is about the Trump mentality; Trump is referenced in the book. These are the big novels. Not necessarily the best writing. But they're not books about people falling in and out of love with each other or whatever.

“In the current climate, there have to be more great books around the corner.”

T2 Trainspotting is out now on general release.