Sick Boy is talking to Renton, but it's clearly a "cut out and keep" moment for the audience of T2, the sequel to Trainspotting.

The creators of T2 had an obvious problem: Why? Why write a sequel to a decades-old classic? When has that ever gone well?

In the words of Jonny Lee Miller, aka Sick Boy: "(Trainspotting the original) struck a chord. There's no way you can do that again. When you score 100 on a test … why would you try and do that again?"

Says Ewan McGregor, aka Renton: "Those moments … are moments in your life, you can't live them over again."

Nevertheless, it got made.

Choose afterlife.

Over an afternoon talking to the team – three of the four main actors plus director Danny Boyle – it becomes clear they made T2 simply because they could. Because two decades ago they had made a film where everything went right – the right source material, the right music, the right attitude, the right time, the right actors, the right director – brilliant talents caught at that moment in their rising arcs where anything was possible.

They had made a film that, for many, was the only film that really mattered in an entire decade (OK, that and Pulp Fiction). A film that bound episodic sketches from the lives of a gang of four Edinburgh heroin addicts into a whole that brimmed with oxymoronic energy. A film with the only soundtrack you would find in absolutely everyone's CD collection. That turned Lust For Life and Born Slippy .NUXX from obscure songs into unlikely anthems, lyrics yelled at parties over a haze of drugs and a sound system buzzing with the strain.

Nostalgia. Literally, the pain of coming home. Often triggered by music.

It was that moment where youth peaked. And now, these 40-something guys couldn't help wonder if there was a little more pixie dust left in the bottle. They wanted another pure, blissful hit of that Trainspotting drug.

Of course, there's a more prosaic story behind T2, too.

Ten years ago they had a script – an adaptation of Porno, Irvine Welsh's sequel to Trainspotting the novel.

"It didn't work and I didn't send it to (the cast)," Boyle says. "I just thought 'they won't do that because I don't wanna do it either'."

In the commercial film world this isn't usually the point where a dumb project is mercifully put out of its misery. This is the point where producers start throwing more money around until everyone grudgingly agrees to what they know is a mistake.

"People say 'well offer him this, offer him that'," Boyle says. Except he didn't. "We just thought no, we're not going to make it like that. And we might lose it entirely. But we should stick to those principles."

And that was that, for almost another decade.

Then Lee Miller got a postcard. On one side, the original Trainspotting poster. On the other a scrawled message: "Dear Jonny. We're having a go at T2. Script soon. Love, Danny."

Like his cast mates, Lee Miller is not hard up for work. Elementary, the American Sherlock-alike, is a hit. But he was drawn to the idea.

"You know I have confidence in someone like (Boyle), I trust his judgment," he says. "And I trust his ability to hear argument and change his mind. So you knew we (still) wouldn't have done it if we didn't all feel it was right."

Around the same time, McGregor ran into Boyle in London. Boyle told him that the next day he, Welsh, Trainspotting screenwriter John Hodge, Trainspotting producer Andrew Macdonald and Boyle's collaborator Christian Colson were heading up to Edinburgh. They were going to stay in what Welsh calls his "Big Brother flat" for a week, brainstorming ideas and locations for T2.

"And after that John Hodge came back and wrote this beautiful script," says McGregor.

Boyle loved it too: "As soon as I read it I thought 'they'll do that, and we'll end up making this film'."

The script uses its biggest problem as its biggest asset. It disarms obvious criticisms by making them the themes of the film: the return to old haunts in search of something that isn't there any more, the trap of falling into old habits and unlearning old lessons, the treacherous lure of nostalgia.

Or as Robert Carlyle, aka Begbie, put it in an interview last year: "the film sort of tells you to think about yourself (and) you're going to be thinking 'f---, what have I done with my life?' "

Now the only obstacle was a logistical headache, of getting busy A-list actors to Edinburgh at the same time.

"I was determined then, that whatever stood in its way I was going to just flatten," says Boyle. "If there was any problem I was just like 'boom'."

They would do it over their summer holidays, in the break between shoots of the stars' TV commitments in the US: Lee Miller's Elementary, McGregor's Fargo and Carlyle's Once Upon a Time. They and Ewen Bremner (Spud) would be paid the same money, plus a cut of the profits.

They clearly weren't doing it for the money, anyway.

Carlyle was so excited about rediscovering Begbie that he lost two front teeth – one a temporary crown that was due out anyway, and its neighbour just because he "thought f--- it, just take that one out as well. My dentist said 'are you insane, we can save this'."

He also cut his hair back to a Begbie style, despite it being a breach of his TV contract. He said "I'm changing it anyway … f--- 'em".

He stayed away from his wife and children for "a couple of months" during the shoot, because Begbie is "so full of rage and nastiness, you don't want to bring that stuff home with you. But you don't want to drop it, you don't want to let it go because trying to get that back every day, you can drop the ball. You have to keep a little bit of that inside you."

And he loved catching up with the cast members, some of whom hadn't seen each other for years.

"We got on really well, we were really close – it was a beautiful moment. We had a meal before filming began – we met up and we were just looking at each other. Seeing who had aged the best."

And who had?

"McGregor."

For Ewan McGregor, T2 was bittersweet – a reminder of films he hadn't had the chance to make with Boyle after their very public falling out.

"Working with Danny was very special after a long time not working with him," he says.

He's alluding to the "feud" between the two, which began when Boyle directed The Beach and cast Leonardo DiCaprio in the lead role. McGregor, who thought the role was his, felt jilted. The pair bumped into each other from time to time but it was like "meeting an old lover or something".

In 2013, Boyle admitted in an interview "we didn't' treat him very well … we gave Ewan the impression we were going to cast him in (The Beach) and we didn't".

Years later, they met, and talked, Boyle told Empire magazine. "I said how sorry I was, the way we had treated him, and it rebuilt from there."

The relationship is still tender, but strong.

"I loved working with him again now," says McGregor. "I think he's one of our greatest filmmakers today. It was just like coming home, it was great."

As well as coming home to Boyle, McGregor felt he was coming home to his character, Renton.

He is annoyed how much has been made of one comment in another interview, saying he was worried that having been away for so many years he didn't feel Scottish enough any more.

"That quote's really misunderstood, it's terrible, I'm so embarrassed," McGregor says. "I haven't lived in Scotland since I was 17 years old (but) I am Scottish. I worried that maybe I wouldn't be able to get in touch with Renton, such an iconically Scottish character.

"'I wasn't worried about the accent, or like being Scottish or anything. It was just an example, of one of my worries of, 'am I going to be able to be Renton again?' It meant a great deal to us, this film, and we were all anxious that we wouldn't f--- it up somehow. It's a sort of odd, unique situation to play somebody that you last played 20 years ago. There's no book that tells you how to do it."

But in the end, he found it easy. The script helped – Renton himself has been living overseas since the end of the last story – he'd visited Scotland less than McGregor, who goes back every year.

"The day I arrived at the T2 set I met Ewen Bremner in the lunch queue and I said 'I'm quite nervous'. And he went 'I was nervous too, and you'll just see when you get on set it's fine'."

He discovered Renton was still in him, in a physical, instinctive way. In one scene he finds himself sprawled on a car windscreen. While shooting the scene he laughed, like in the first film, without thinking.

"It just happened, like a memory, a muscle memory."

In another scene he's pushing up through the floor of an attic, through the rafters. As he rehearsed it his body "remembered" the scene emerging from the toilet in the original film.

"There was something I did with my head coming up the toilet (and in T2) I went 'oh'. I just found myself doing it, like it was a Renton-y place, you know?"

Nostalgia, again, that unconscious return, like an ache.

"(Shooting T2) made me very nostalgic for lots of reasons," says McGregor. "The experience of making (Trainspotting) was so extraordinary and sort of as good as it could be. The filmmaking of it was amazing. The subject matter, the actors, the set design, the cinematography, the music, the feeling that we created on set, knowing that we were making something amazing with Danny was … extraordinary.

"It was an amazing thing for all of us."

T2 Trainspotting opens on February 23.