In the new film, Bening plays Diana Nyad, the swimmer who once swam from Cuba to Florida.
Annette Bening is shedding light on the training schedule she underwent to transform into Diana Nyad for her new film, Nyad.
Talking with ET's Denny Directo, the 65-year-old actress said she cried in the pool alone while feeling overwhelmed by the task of portraying the world-champion marathon swimmer.
The film -- which stars Bening as Nyad and Jodie Foster as her swimming coach Bonnie Stoll -- follows the inspiring true story of Nyad setting out at the age of 64 to swim from Cuba to Florida.
To get into character, Bening said she trained with an Olympian for a year to get into shape to take on the role of the woman who swam 100 miles (53 hours consecutively) across the Straits of Florida.
"I didn't cry in front of Jodie Foster because I was so intimidated," Bening said. "But I cried in the pool, by myself."
"I just thought, 'Oh, I'll just jump in, I'll just start swimming and I can figure this out,' and then I realized, 'Oh man, this is like a thing,'" she said of the intense training she went through prior to and during filming.
The Oscar-nominated actress said through it all, she aimed to make Diana and others like her proud.
"I wanted to do Diana justice, I wanted to do real swimmers justice. I worked really hard at it and that of course, like anything, when you work really hard at something -- there's this great feeling that comes from trying and trying," she said.
The initiative paid off, Bening said, as when they finally got to the Dominican Republic to film, the actress was prepared and was able to swim "lap after lap" and making the photographers' jobs easier.
Bening could not quantify exactly how many hours she spent in the water while filming but said some days could go on for five or six hours a piece with breaks here and there.
"It's such a demanding thing to do but it's also very deeply -- it relaxes the central nervous system," she said of being in water all day. "That's why people who are on the water tend to be kind of mellow. Except Diana Nyad, she's not mellow. She's the one person who's never mellow."
Since leaving filming behind, Bening said she still swims every day and that it has become a great stress reliever in her life.
"I have to, or else -- you know, I start to chew my clothing," Bening joked.
Nyad is now in select theaters nationwide and streaming on Netflix.
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