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“Nothing’s more miserable than the set of a comedy,” declares Colin Firth when he sits down with PEOPLE to look back on 30 years of charming audiences, most often with self-deprecating wit. “It’s rampant with anxiety as you try to be funny.” Not that he ever looks as if he’s trying.

At 53, the star of Bridget Jones’s Diary and The King’s Speech (for which he won an Oscar) knows how to win hearts – and get laughs. But his latest role is darker fare. In The Railway Man Firth plays Eric Lomax, a real WWII British Army officer tortured by the Japanese while working on the Burma railroad. “More than 100,000 died” in the effort, he says. “Why aren’t we hearing about their heroism?”

Tough stuff, but Firth and costar Nicole Kidman buoyed each other’s spirits. “We weren’t rolling in the aisles, but there was relief.” And, thinking it over, Firth allows that keeping things light isn’t so bad after all. “What I said about comedy was complete nonsense,” he decides, looking at a photo of himself with Mamma Mia! pals Pierce Brosnan and Stellan Skarsgard. “We had a whale of a time. I said, ‘Do you ever sometimes feel overpaid?’ They nearly threw me in the sea.”

Underscoring his Mr. Darcy-ness, Firth played Helen Fielding’s version of the dashing suitor in the two movie versions of her bestsellers. But (spoiler alert!) the author kills off the character in her latest book.

That doesn’t worry Firth, who hasn’t ruled out reuniting the cast (Renée Zellweger and Hugh Grant). “It would be like herding cats,” he says, adding, “I’ve always thought it would be more interesting to do When we’ve all deteriorated a bit – a story of a different generation.”