Blake Lively says her latest movie role wasn't just another acting gig, but an "athletic event."
ET caught up with the 28-year-old actress at the junket for her new film, The Shallows, in New York City on Wednesday, where she revealed how she got her body bikini ready for the thriller just ten months after giving birth.
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In the film, Lively -- who welcomed her first child, daughter James, with husband Ryan Reynolds in December 2014 -- portrays the role of Nancy, a surfer who finds herself clinging to a rock and fighting for survival 200 yards from a secluded shore when a great white shark starts circling her. That means a wardrobe full of bikinis, wetsuits and beachwear, which required her to get fit fast!
"Well, you know, I had an amazing trainer," Lively told ET's Carly Steel. "I was fortunate to have Don Saladino help me out and he just kicked my butt in the best way, in the most healthy way."
Lively explained that the cast and crew shot the movie for eight weeks in Australia, so it was important that she was not just in shape "for the sake of being in a bikini," but also to keep up with the physicality of her character's badass role.
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"Being in waves like that, swimming like that, doing such long takes like that all the time -- I became so much stronger and more fit by the end of production. I was working out 13 hours a day because shooting was working out," she dished. "It was neat to be able to do that, and it was neat to have that challenge after having a baby, because you think your body is so different -- you think no matter what, it's never [going to be] totally be the same."
"And to be able to be in better shape than I had ever been in before," she continued, "I thought, 'OK, that's really encouraging.'"
Lively was also encouraged by her husband, who is no stranger to taking on tough action roles. She told ET she was specifically inspired by Reynolds' performance in Buried, in which he plays a U.S. truck driver fighting for survival when he discovers he was buried alive in Iraq.
"He always talked about Buried and what an important thing that was for him to be able to do," she explained. "What a challenge it was and boy, was it a payoff."
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Lively admitted that Reynolds actually read the script for The Shallows before she did, giving it his instant stamp of approval.
"He was very kind and generous to do that," she said. "I was really sick [at the time] and I was working, and so he read it and he was like, 'Aww the action is amazing.'"
"[For this film] he said, 'Be prepared, this movie is a physical challenge, it is an athletic event,'" she continued, noting that her husband said prepping for the role would be a lot "like training for a marathon."
In addition to her film hitting theaters on Friday, Lively is also gearing up for the arrival of her and Reynolds' second child -- but that hasn't stopped her from turning heads on the red carpet!
WATCH: Blake Lively Talks Maternity Fashion at 'The Shallows' Premiere
"I'm really lucky to have access to really great designers [and] I like mixing things that are unexpected," she said of her chic maternity fashion choices.
To hear more, watch the video below.