The reunion of Ewan McGregor with his one-time regular collaborator, director Danny Boyle, was confirmed once more this weekend in London, as McGregor hosted a private screening of his directorial debut American Pastoral, based on the Philip Roth novel, at the tony Bulgari Hotel. Boyle then chaired a spirited Q&A for the friends and family audience – which included McGregor’s parents and his uncle, Star Wars star Denis Lawson – at which the pair swapped war stories, and McGregor expounded on the lessons he’d learnt from the man who’d directed some of his most defining early roles.

“We didn’t have the the rehearsal time that we did on Shallow Grave and Trainspotting,” McGregor said of his project. “But once we started, I’d clear the set, like you do, and go over the scenes alone [with the actors]. You’d be amazed how obvious that sounds, but how often it isn’t done. But I learned that from you, and that was how we went about it in Shallow Grave and Trainspotting.”

“I was very lucky to see the film while you were cutting it,” Boyle noted. “I was crying at the end.” McGregor said he’d listened to the advice he’d received from Boyle and changed the edit accordingly.

The pair made three films together in the late ’90s, but split when McGregor was passed over for the lead in Boyle’s adaptation of The Beach; the role went to Leonardo DiCaprio. Earlier this year, McGregor expressed regret for their time apart. “Eighteen years since that point, I just regret all the films that we didn’t make together, me and Danny,” he had said.

They were finally reunited on set last year, collaborating on a sequel to Trainspotting. “I worked with Danny immediately after [American Pastoral],” McGregor told Deadline. “I did my last day in the sound mix for American Pastoral on a Friday and I flew to Scotland on a Monday to start on Trainspotting 2.

“When I arrived on set, I wondered what it would feel like, because when you’re directing and acting you never have a minute. You’re always on the move; your brain and your body. But it was an extraordinary situation, because we were revisiting Trainspotting and I was playing Renton again, which was great and a joy. I had coffee in my trailer and read the newspaper everyday. It was quite good.”

The experience, he said, had taught him to go easier on directors faced with pressures that go beyond the scene being shot. “I always felt like a filmmaker anyway, because if you take an interest in how the film’s being put together and you like to work with the crew, and you’re aware of what the camera is doing and why it’s doing it, then you are a filmmaker really.”

McGregor also expounded on the European press tour he’s taken with American Pastoral, where he was frequently questioned about his interest in adapting an American novel. “Europeans are very keen on this idea that as a Scotsman I must have had a really good angle on American politics. I was like, ‘No, no, that’s not right.’

“‘Why as a Scottish person do you think you’re the right person to make this movie?’ [journalists asked]. I kept telling them about you,” McGregor said, addressing Boyle. “I kept saying, ‘Danny Boyle’s not Scottish, but he directed one of the most Scottish films in the world.”