Along with the grin is his dominant characteristic, geniality – a boyish enthusiasm, for film, family, motorbikes, charity work – whatever he talks about, and McGregor talks a mile a minute.

After more than 20 years as an actor, 45-year old McGregor has directed his first film, American Pastoral, based on the Philip Roth novel, in which he also stars, and he has been on a whistle-stop tour of Europe, talking it up.

"My eyes are still being opened to the other side of the curtain," he says with a laugh. "Having to sit with producers and talk about distribution and marketing and reviews and all this shit..." He sighs. "I thought I understood the film business, but I'm realising I've still got a lot to learn."

Set in the 1960s, American Pastoral tells the story of Seymour "Swede" Levov, a Jewish businessman whose seemingly golden and imperturbable middle-class life is violently disrupted when his 16-year-old daughter, Merry, gets swept up in radical politics and blows up the local post office, before going on the run.

As a film, American Pastoral has had a difficult history. McGregor was first approached to play the part of Swede some six years ago, but the script, by John Romano, was circulated through a number of directors without ever getting off the ground. In 2014, McGregor was in New York, stuck in traffic, on his way to perform in Tom Stoppard's play The Real Thing, when he got a call offering him the job of director. He was thunderstruck. He recalls spending a day with the script, envisioning himself going through every step of the acting – and directing – process, asking "Can I do it? Can I do it? Can I do it?", finally concluding that, yes, he could.

Roth's work has proved notoriously problematic to film in the past. So much of its power lies in the writing, with its great unspooling meditative sentences and digressions (huge tracts of American Pastoral are given over to descriptions of the techniques and history of making leather gloves; fascinating in Roth's telling, but hardly cinematically compelling). The film is therefore necessarily a work of boiling down – even more so given that the original budget of $30 million was cut, as McGregor points out, "because I was a first-time director", taking several scenes from the original script with it. The film has had a mixed response from critics, but it does a highly creditable job of conveying the drama of Roth's book, conjuring a strong sense of period and place, and drawing some powerful performances from Jennifer Connelly, as Swede's wife, Dakota Fanning as Merry, and McGregor himself as Swede. McGregor has not met Roth, but the author has let it be known that he likes the film.

McGregor says he has always felt like a filmmaker; having acted in 55 films over the past 22 years, he has been around enough sets and directors to get an idea of how it works.

McGregor moved from London to Los Angeles eight years ago. He was spending a lot of time in America, and it seemed to make sense to move there, although ironically few films are made in Hollywood, and he has little to do with the film community there.

A goodwill ambassador for Unicef, he was recently in Iraq, visiting refugee camps. "Somebody asked me yesterday, 'Did you feel you needed to go to Iraq to get out of the Hollywood bubble?' I don't live in a Hollywood bubble. There's this idea that living in LA is a bit like being in a 24-hour hip-hop video. I've never been to a hip-hop party in my life." He laughs. "I'm waiting for the invitation."

Brentwood, the area where he lives, is suburban, in both geography and temperament.

"Our life totally revolves around our kids," McGregor says. "It's children, school … the kids' friends' parents become your acquaintances and sometimes your friends. I like it."

McGregor grew up in the small Scottish town of Crief. His father, James, was a PE teacher, his mother, Carol, a special-needs teacher. An elder brother, Colin, became an RAF Tornado fighter pilot. McGregor's ambition to act was largely inspired by the example of his uncle, the actor Denis Lawson, whom he once recalled coming back up from London on a visit, "sheepskin waistcoat, no shoes and beads and stuff. I'd be, like, 'I want to be that guy.' And the fact that he came from the same small town made it possible."

With his parents' encouragement, he left Morrison's Academy at the age of 16 to join the Perth Repertory Theatre as a stagehand. He went on to study drama at Kirkcaldy College in Fife, before enrolling at London's Guildhall School of Music and Drama for a three-year course.

His first notable role was in Shallow Grave (1994), directed by Danny Boyle, written by John Hodge and produced by Andrew Macdonald. The same team would go on to make Trainspotting (1996), a scabrous look at a group of heroin addicts in Edinburgh based on the Irvine Welsh novel, in which McGregor starred as Renton. The film, which was nominated for a best screenwriting Oscar, and won a slew of other awards around the world, made stars of its four principals – McGregor, Ewen Bremner, Jonny Lee Miller and Robert Carlyle – and established Boyle in the vanguard of British directors.

Lauded as one of the best British films of the 1990s, when Welsh published a sequel to Trainspotting, Porno, in 2002, repositioning Renton et al 10 years on in the porn industry, expectation was high that Boyle, McGregor and the other Trainspotting principals would be reuniting to make a film of it.

Fourteen years on, it is finally happening – but not without a river of tears of recrimination, and reconciliation, in the meantime.

Following Trainspotting, McGregor worked with Boyle and McDonald once more on A Life Less Ordinary (1997), a black comedy in which McGregor co-starred with Cameron Diaz as a janitor who gets fired and kidnaps his boss's daughter. But he fell out badly with the director after Boyle cast Leonardo DiCaprio in the film The Beach (2000) – a decision that McGregor found deeply wounding.

"I felt at that time that as an actor I was part of the new wave of something," he says. "I was lucky to be Danny's actor, and I felt part of something that was bigger than all our careers with Shallow Grave and Trainspotting. I felt we'd left our mark on British cinema.

"So when we were approaching The Beach, I thought I was playing that role and I was encouraged to think so; I was told for many months I would be doing it; and then I was told I wouldn't. And I was mystified and it had been handled badly, and whatever. But it hurt me very much at the time. I was rocked a bit, and my confidence was dented. And I didn't quite understand why they went down that path when I thought we were about something else. I understood when it came down to dollars. But it was a Hollywood version of something we weren't really about."

After that, he says, he and Boyle "didn't speak to each other for years and years".

In 2002, when Porno was published, he was sounded out about appearing in a film version. "There wasn't a script at that point, but I wrote to them saying I didn't want to do a sequel to Trainspotting. I didn't like the novel Porno very much. It didn't move me like the novel Trainspotting had, and I didn't want to tarnish the reputation of Trainspotting by making a poor sequel."

Over the years, he says, he would bump into Boyle here and there, and "even though I should have felt cross, I was always delighted to see him. We were never mates. It was a professional partnership. We didn't hang out in each other's homes or anything like that. But the truth of the matter is that I love Danny: he was my first director – which is a bit like being your first love, I guess, because I didn't know what it was like to be a film actor until I was with him. When I was on set with him, I would look over and see him and feel genuinely happy that he was there. I'd do anything he asked.

"I felt – and still feel – that he directs in a way that is wonderfully unusual for an actor; he knows what you're about, and he took you forward in a way that led to some really extraordinary work, that you look back and go – 'F--ing hell, I didn't know I could do that.'"

In 2009, at a ceremony in Los Angeles, along with the Slumdog Millionaire star Dev Patel, McGregor presented Boyle with the John Schlesinger Britannia Award for Artistic Excellence in Directing.

"I asked, 'is it televised?'," he remembers. "Because they always write you something to say and it's always crap. But they said, no, so I said I'll write my own thing, and there's no need to put it on autocue. So I got there, and there were cameras everywhere! I was going on after De Niro and before Governor Schwarzenegger. I was terrified. But on I went. And because it wasn't on autocue I just said it to Danny, sitting there at his table, and I talked about how it felt to be on his set and how I felt defined as an actor by being his actor. And at the end I just said that I loved him and I missed him. And then Danny got up and talked about how his job was all about the actors, and he was lucky to have two such actors on the stage to give him this award, and he thanked Dev, and 'Ewan, whose graciousness I don't deserve'. And I just felt, that's it. It was done."

With the 20th anniversary of Trainspotting coming up, the idea of a sequel took on greater momentum. John Hodge delivered a script that everybody was happy with, and on a Friday last August, McGregor completed the final day on the sound mix of American Pastoral and on the following Monday flew to Scotland to begin shooting the film, titled T2: Trainspotting. McGregor will say only that the script is "very loosely based on Porno. And I can't tell you any more". He laughs. "I wouldn't want to spoil it."

When he's not working, he gets to be at home with the family and take the kids to school in what he calls "these silly old cars" – any one of a collection of old Volkswagens or his black 1960 Rolls-Royce Silver Cloud. "I drive around LA in this big old, silly old Rolls, like Dudley Moore. It's the daily school-run car. I love it – just love it. And then when they're at school and I come back and it's, right, which bike is it going to be? And I'm off…" He gives that big McGregor grin. His trademark, which suggests he hasn't a care in the world. "Oh, I have. I've got three people to get to school in the morning. I'm always wondering, where are their f--ing socks?"

Telegraph Magazine, London

American Pastoral is out now; T2:Trainspotting is out on January 27