Edinburgh has changed in the two decades since Danny Boyle's Trainspotting advertised it as a perfectly awful place to shoot heroin. The gamblers' squat that housed the Worst Toilet in Scotland was torn down for luxury apartments. The same goes for the nightclub where Ewan McGregor picked up an underage schoolgirl. The pub where Begbie broke a pool cue over a stranger's back was upgraded twice: first to a whiskey bar, and then to a hipster arcade.

"The vibrancy of the city is completely different," says Boyle on a bright afternoon in Los Angeles. In 1996, his needles-and-cobblestones Edinburgh joy buzzer was such a shock to the film industry that it felt like it was shot on a different planet. Boyle even had to rerecord its clanging Scottish brogue narration so Americans could understand it. They did, and the film — and its soundtrack — made millions around the world. Sneered Trainspotting's posters, "Hollywood come in ... your time is up."

Today, the two towns share a global hive mind along with their disparate versions of the English language. "Gentrification," nods Boyle. "Edinburgh's full of young people now." But when Boyle and the cast returned to film a sequel, T2 Trainspotting, Edinburgh made room for a few older people too. The former bad boys of British cinema have become global statesmen. McGregor moved to Hollywood and starred in three Star Wars movies. Boyle racked up so much acclaim with movies like 127 Hours, Steve Jobs, and Slumdog Millionaire that he was entrusted with the U.K.'s splashiest gig: helming the opening ceremony of London's 2012 Olympics, where he even directed the Queen. Laughs Boyle, "They hang medals on you, eventually."

Straightaway, the new Edinburgh was a shock. Cheerful girls in mini-kilts stood outside the airport handing out maps, yet the lassies were from Poland. The European Union, which was in its infancy when Boyle made the first film, had opened up the country, so much so that in the middle of T2's shoot, England chose to Brexit. Scotland voted to stay. "The Scots are no fools," says Boyle. "I think Scotland will leave the U.K."

Boyle found the culture clash so surreal that he put a welcome-home scene in the script. It goes like this: When McGregor's Mark Renton finally returns to Edinburgh after 20 years in Amsterdam, he's greeted by a long-legged Slovenian. Renton's self-imposed exile was for his own safety; there, he quit heroin and hid out from his spiteful mates Begbie (Robert Carlyle), Sick Boy (Jonny Lee Miller), and Spud (Ewen Bremner), who haven't forgiven him for stealing £16,000 at the end of the last film. Renton's brought apology cash, but when he tosses an envelope of bills at Sick Boy, now a two-bit porn peddler who manages an empty bar, his ex-friend snarls, "What am I supposed to do with that? Buy a fucking time machine?"

The Trainspotting boys have grown up, but they haven't matured. Bleached-blond Sick Boy's become the geezer at the club, and his young Bulgarian girlfriend Veronika (Anjela Nedyalkova), a prostitute, doesn't respect him. "His hair is falling out, but he's dyeing it still," tuts Boyle. "That's just sad. Women get such flak for it, but they're so much better at aging than men. Men just deny it: 'What do you mean I'm old?!'"

Psychopath Begbie, now an ex-con calling himself Franco, has a timid son and a penis that doesn't work. Now Begbie’s drug of choice is Viagra. And smack fiend Spud struggles to afford his next hit from the snarling teens in the stairwell, who can't imagine that this skeletal wraith was once wild like them. The last time audiences saw Spud, he was thrilled that Renton had slipped him £4,000. But the gift was a curse: Spud spent the money on smack. Heroin addicts don't get happy endings. "It's astonishing that he survived to get to that age," says Boyle. "Most of them don't."

"In the first film, we benefited so much from their recklessness and carelessness, the fact that they didn't even acknowledge time," says Boyle. "And then you make a story about them now and you realize that they are beginning to realize that time doesn't care about them."

Boyle wanted to make his sequel 10 years ago to pair with Trainspotting author Irvine Welsh's follow-up book, Porno. The problem was his actors. Bad decisions needed to show on their faces, but his vain cast wouldn't age. Cool kids like Angelina Jolie's ex-husband Miller might swagger like they're out boozing all night, but "most of them are tucked up in bed," says Boyle. “Actors always give the impression that they are wild,” he sighs. “I remember we were thinking, You won't look any different, you'll look just the same, so there won't be any reason to make the film.” Waiting for Miller and McGregor to get crow's-feet took another decade.

Plus, McGregor refused to work with him. The two men launched each other's careers with a string of ’90s indie hits: Shallow Grave, Trainspotting, and A Life Less Ordinary, in which McGregor got to smooch Cameron Diaz. Then they had a public breakup when Boyle gave McGregor's part in The Beach to Titanic golden boy Leonardo DiCaprio. McGregor didn't speak to Boyle for years, much like how Renton dodged his mates. The chill began to thaw at the 2009 BAFTA awards when McGregor presented Boyle's Best Director statuette for Slumdog Millionaire, adding, "I love you and miss you.”

Technically, the two still haven't hashed out their feud. "Men are terrible about actually talking to each other emotionally," says Boyle. "We just gave each other a hug and got on with it."

After all these years, standing with McGregor in Renton's bedroom, the hovel where a baby corpse crawled on the ceiling, was "bizarre." So much has happened that even the stack of records is a dirge. David Bowie, gone. When Renton slips on Iggy Pop's "Lust for Life," the first chord is so loud, and so painful, that he turns it off. Boyle had rewatched the original Trainspotting, of course. ("Not too much," he stresses. "You get slavish, you get inhibited.") He wanted to pay homage to the camera moves and a handful of shots, even though that meant unlearning some of the tricks he'd picked up making more expensive movies. He's at peace with the dead infant's lumpen, pre-CG face — which shows the film's age as much as any wrinkle. "Now films are so much more sophisticated," says Boyle. "You wouldn't get that fault. But I love that fault."

More than anything, though, Boyle tried to capture Trainspotting's manic energy. Instead of planning out every detail in advance, he'd just get everyone in a room and brainstorm. One afternoon near the end of the shoot, he and McGregor were back in that claustrophobic, train-wallpapered room when his old friend asked him, "Shall I dance?" He put the Iggy back on and cut loose. Recalls Boyle, "I said to him, 'That's it for me. If I get knocked down by a bus and that's the last thing I've shot, of you dancing to Iggy Pop in your childhood bedroom, I'll die a happy man.'"

As for the dead baby herself — or really, the twins who played her — she's fine. Boyle tracked them down on Facebook, "just in case they turned into a drug addict or they killed someone." Today the girls are 22, and they're lovely, with big, wide smiles. Boyle takes out his phone to show a picture of them, beaming with pride. "One was in PR, the other was in finance, I think," he says. He invited them to T2's premiere, where the three finally had their first conversation. "Isn't it weird?!" grins Boyle. "They went around the party introducing themselves by saying, 'We're the dead baby!'"

The twins are a reminder that right now, somewhere, there's a kid who was a newborn when Trainspotting opened. To them, Trainspotting is from the musty past. There's no triumph in hearing Renton brag that someday he'll buy a compact disc player. (Um, sure? Scour a flea market maybe?) Instead, in T2, Renton chants, "Choose Facebook, choose Snapchat, choose Twitter, choose live-blogging from your first wank to your last breath," while he and Boyle and the audience know that that pledge, too, will eventually be passé.

"Cinema is vampiric," says Boyle. "It needs young blood, young energy, young life." Now it's someone else's turn to shake up the world.