British actor Ewan McGregor made his name in the 1990s working with screenwriter John Hodge and director Danny Boyle on the much-loved triumvirate of Shallow Grave (1994), Trainspotting (1996) and A Life Less Ordinary (1997).

Then he severed his ties with them when they made The Beach (2000).

Boyle had indicated to McGregor that the Scotsman would get the leading role in The Beach. As it transpired, Boyle made the film with Leonardo DiCaprio instead and the relationship between the filmmaker and McGregor derailed.

They have now reunited for T2 Trainspotting, the sequel to the most iconic of their films together, which opens in Singapore tomorrow.

"The fact that we have now worked together again is down to the graciousness of Ewan McGregor because we didn't behave very well towards him," Boyle, 60, concedes.

"The way we behaved around The Beach was not something that I am very proud of, but Ewan was very gracious in forgiving me that. And it is wonderful to be working with him again. I am very lucky."

McGregor, 45, says that while he expected to get the lead role in The Beach all those years ago, it was not so much the fact that he lost out, but more in the way the situation was handled. What he regrets most, he notes, is that he and Boyle missed out on making many more films together across the following years.

"As Danny and I had got on so well, that was what upset us with what happened on The Beach," he says. "I loved being Danny's actor so much. Now, though, all of that stuff with The Beach has long since passed under the bridge."

Of course, returning to such a beloved film was a huge risk for McGregor, Boyle and Hodge, as well as for the other principals from the original Trainspotting cast - Robert Carlyle, Ewen Bremner and Jonny Lee Miller.

Made on a £2 million (S$3.5 million) budget, Trainspotting outgrew its modest indie roots to become a bona fide cultural phenomenon. It is one of the few films that defined 1990s Britain and people still talk about the characters of Renton (McGregor), Spud (Bremner), Sick Boy (Miller) and the terrifying Begbie (Carlyle) to this day.

"We were very aware of and respectful of the love for the first film," says McGregor. "We all felt like that. None of us would have done a sequel for the sake of it. But when we were presented with John Hodge's script for T2, it was very clear that there was no need to worry.

"It was so good to see these characters again and see where they were at. The film has something to say, as the first one did in the 1990s. It explores something quite deep, about people looking back on their youth."

According to Bremner, 45, the first day on set was "quite emotional, almost surreal, for all of us". "I think we all felt quite tender. We were given such a gift 20 years ago with these parts," he says.

T2 picks up two decades after the original film ended and the audience is shown how time has not been kind to them.

Boyle says: "It's 21 years since the release of the first film and conventional wisdom says that 20 years is too late to do a sequel. But actually, the time lapse gives the film a raison d'etre. When you put the actors side by side with how they looked 20 years ago, it's brutal. We looked at it 10 years ago and the actors didn't look that different. I used to joke with them that they must be moisturising all the time.

"Twenty years is a long time and you can feel it."

Miller, 44, agrees, noting that life experience has given the characters added depth and plenty of pathos.

"The important thing was that we waited until it got to 20 years because then you can ask some interesting questions about relationships and life," he says. "The characters in the original didn't give a damn. They were just going for it and experiencing life head on. But you don't feel the same way 20 years on. You've got all this baggage and these problems and this cynicism.

"There was no point in making a sequel to Trainspotting unless we were examining some bigger issues: What's it like being older? What have you done? What's happened to the characters and what are the implications?"

That time has been harsh on the four male characters is emphasised by the agency of the women in the film, not least Diane (Kelly Macdonald), who had a relationship with Renton in the first movie.

In T2, she has gone on to become a lawyer. "Diane has done a wonderful job in the meantime and her becoming a successful lawyer stands testimony to how much time the boys have wasted," says Boyle.

As one might expect, the new film is full of knowing nods to the original. There is a scene in a toilet, another where audiences see Renton bouncing off the bonnet of the car, and Renton and Spud running together - though the vast majority of these moments grew organically during filming, according to the director.

The only intentional reference written into Hodge's script arrives when Spud leaves a boxing club and we see him under the huge arch that features in the opening to the original film, just as Renton is hit by a car. "That was built into the script," says Boyle. "In fact, it was absolutely fundamental."

Will references from Trainspotting and T2 appear in a third film?

"Stranger things happened," says Boyle with a smile. "If you had asked me about a sequel after the first one we would have gone 'Whoa!'"

•T2 Trainspotting opens in Singapore tomorrow.