LONDON — The twist in “Bridget Jones’s Baby” (opening Friday) is that Bridget doesn’t know if the father of the baby she’s carrying is Mark Darcy (Colin Firth) or Jack (Patrick Dempsey, who replaces Hugh Grant in this third film). Renee Zellweger is back playing Bridget for the first time since “Bridget Jones: The Edge of Reason” in 2004.

“I was excited about coming back to it all, particularly because I got to again wear those big girl panties,” said the actress with a laugh. “It also was so much fun, because I got to work with these guys,” she added, pointing to Firth and Dempsey, who joined her for our interview.

Returning to the role she’s played twice before — but not for a dozen years — “was a process,” she said. “Regaining the accent and all that was familiar to me this time around. However, I did work with some dialect people. … In many ways, it was like developing any character you’d do in a film — even if you’ve played her before.”

As for Firth, he said he was “surprised how happy I was to return to playing Mark Darcy again. I haven’t gone back to the same role very often in my life. I’ve described that process as somewhat like putting on a pair of wet swimming trunks, and I was wondering if that was what it was going to feel like in this case.

“But that didn’t happen here. It was actually very uplifting. I tried to feel cynical and pessimistic and gloomy about it, because that’s sort of what I do,” he said with a wink. “But it was difficult to do that, because the people — and the combinations of people back together — were such fun to be around!

“In many ways, it was quite moving. It was like a family reunion — but the good version of a family reunion,” Firth added, as Zellweger and Dempsey laughed heartily. “Oh yes, and Patrick was my new best friend.”

Not surprisingly, as the new addition to the “Bridget Jones” franchise, Dempsey admitted “I was a little nervous in the beginning, because of the history and the success of the films and the characters in the novels and all that.

“But about a week or so into the filming, I began to relax a little bit and started to enjoy it — largely because we all got along so well,” he noted, turning to his co-stars.

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The subject of chemistry is a hard thing to really put into words, but all three actors made a point of saying they did feel it from the very beginning of filming. Naturally, Zellweger and Firth drew on their previous collaborations, but the actress said, “Patrick was such a seamless addition to our little group. There was a real bromance going on between Colin and Patrick, and I was lucky to be the third part of that.”

Mentioning Dempsey’s well-known passion for auto racing, Zellweger laughed as she said, “Patrick would go off racing somewhere, and then he’d come back — and there he would be with Colin off in a corner talking all excitedly about it. … And then we’d eventually get back to filming!”

Yet, Firth said he felt that Zellweger was the core of the reason the trio had the chemistry they did on screen, “which hopefully will translate for the audiences.” Turning to his fellow Oscar winner, Firth said, “You cheer-led the whole thing and that was a big part of why it felt so alive to me. That set the tone.”