EXCLUSIVE: Renee Zellweger Talks Preparing to Be a Mom for 'Bridget Jones's Baby': 'Those Little Guys Are Heav

ET sits down with Renee Zellweger, Colin Firth, and Patrick Dempsey.

Renee Zellweger may not be putting on the pounds this time to play the lovable Bridget Jones in Bridget Jones's Baby, but she is still stepping out of her comfort zone in another big way -- playing a first-time mother.

ET sat down with Zellweger and actors Colin Firth and Patrick Dempsey in London on Tuesday to talk about the highly anticipated third installment of the franchise, in which Bridget is pregnant with a baby, but doesn't know whether the father is Mark Darcy (Firth) or American businessman Jack Qwant (Dempsey). Zellweger, 47, strapped on a baby bump for the role, and says working with the children in the film was her favorite memory of filming in London.

"The children made me laugh," Zellweger exclusively tells ET's Carly Steel. "Adorable and sweet -- so sweet. That little Rafferty [Railton], he killed me."

WATCH: Check Out Renee Zellweger Sporting a Baby Bump on the Set of 'Bridget Jones's Baby'

Still, Zellweger is honest when asked if the role made her feel "maternal."

"Oh gosh, no. It just made me know I needed to get back to the gym and start using some dumbbells because those little guys are heavy," she jokes. "I had no idea. Kudos to the moms."

It's been 15 years since Bridget Jones's Diary first came out in 2001, and Zellweger says she's glad that plenty of time has passed since the second film of the franchise -- Bridget Jones: The Edge of Reason -- hit theaters in 2004.

"It's kind of fun to catch up," she explains. "I was so happy to be back in her world and be reunited with her friends and her family, and it just reminded me of how much I love these characters."

"It really did add a whole dimension, for comedy value," Firth, 55, adds, "because you got a bit of nostalgia at play. Also, the audience has aged with us."

Of course, the film has some throwbacks to the earlier films, including Bridget's love for granny panties. But Zellweger says nobody has ever asked her to wear a pair in real life.

"Oh gosh, no!" Zellweger says. "No, can you imagine? Especially a boyfriend!"

... But is newcomer Dempsey, 50, a fan of the granny panties?

"As much as anyone can be," he laughs.

NEWS: Renee Zellweger Reveals Why She Came Back to Hollywood After 6 Years Away -- 'I Felt Ready'

Bridget Jones's Baby
hits theaters Sept. 16.

Watch the video below to see Zellweger and Dempsey looking adorable together posing for pics in Australia earlier this month, when the two officially embarked on the film's press tour.