How Guy Pearce found his motivation playing British spy traitor Kim Philby

Guy Pearce spent six months playing Kim Philby in “A Spy Among Friends” — and said he still can’t quite understand what motivated the infamous British intelligence officer who fled to Russia in 1963 after decades as a double-agent for the Soviet Union.

Philby’s betrayal — not only of his country, but of his friendship with fellow MI6 colleague Nicholas Elliott (Damien Lewis) — drives the narrative arc of “A Spy Among Friends,” airing Sundays on MGM+ (10 p.m.).

“One part of me feels I know [Philby], to a degree, and there’s another part of me that goes, ‘I have no idea,’” Pearce, 55, told The Post. “One the one hand, yes, I got a grasp of Philby … but he was always sort of slipping through my fingers.

“I kept vacillating between trying to simplify things and go, ‘Well, I guess he was just on the fence — on one level he did believe in Communism and on another level he was embedded in the British ruling structure but had a disdain for it so he simply played both sides.’

“And then I think, ‘Nah, it’s not as simple as that.’”

The six-part series, based on Ben Macintyre’s novel, opens in 1963, when Elliott, whose relationship with Philby dates back to 1940, learns of his best friend’s decades-long deceit — and convinces his bosses at MI6 (also known as SIS) to send him to Lebanon, where Philby, previously exonerated of any wrongdoing, is still spying for Moscow. Their four-day, face-to-face meeting — and what Elliott said to Philby on a hotel balcony, out of earshot of recording devices — is a central motif as the series progresses, jumping back and forth in time to track the roots of their friendship and Philby’s treachery (he spent several years in Washington, DC in the late ’40s/early ’50s).

“I think Philby, in the end, probably felt most hurt by the breakdown of that friendship,” said Pearce (“Mare of Easttown,” “LA Confidental”). “I think he truly believed that Nicholas knew and would understand why he did what he did, but Elliott wouldn’t give him the satisfaction of admitting that. Fair enough.

“It was a really fascinating piece to work on and the question of their immediate friendship and the strength of that friendship was certainly undeniable,” he said. “In the face of what was at stake for them something had to give — and, in the end, it was their friendship. And that’s what makes the story so tragic, because they had a deep love for each other and, like family members, the betrayal is therefore extra-strong and devastating.

“There’s a bit of footage of Nicholas Elliott being interviewed on a BBC talk show after Philby died and there’s a snippet of him saying, ‘Philby wasn’t a double-agent, he was just an outright traitor,’” Pearce said. “You see the hurt in him still … I would love to have understood what it must have been like for Elliott in the years after that — as well as the other people who were tricked.”

Co-stars include Anna Maxwell Martin as Lily Thomas, a lie-detector expert promoted to a key role in the Philby investigation; Stephen Kunken as CIA veteran James Angleton; and Anastasia Hille as Flora Solomon, Philby’s longtime friend who introduced him to his second wife (there’s a lot more to the Philby-Solomon connection laid bare in “A Spy Among Friends”).

Pearce had never worked with Lewis (who’s returning to “Billions”) before “A Spy Among Friends” but was a big fan of Lewis’ late wife, Helen McCrory, with whom he worked on “The Count of Monte Cristo” in 2002. “She and I got on very well — she was fantastic, sharp and funny and didn’t suffer fools,” he said. I was like, ‘Wow, you’re really cool.’

“I felt an instant, tender connection [to Lewis]. He said, ‘[Helen] told me you guys got on really well’ … he’s a brilliant guy, just divine: he’s smart, fun, sweet, gorgeous and also doesn’t suffer fools. We didn’t need to spend time to work out how [Philby and Elliott] would be friends.

“We just clicked.”

Philby spent 25 years in the Soviet Union, where his life didn’t turn out as he envisioned it. (He thought he would be made a colonel in the KGB. He wasn’t.) He died in Moscow in 1988 at the age of 76 and was given a hero’s funeral.

“They didn’t let him in the KGB offices for 14 years,” Pearce said. “He just drank himself to death.”