'The Help's' Jessica Chastain: 'Torture' Gaining 15 Pounds for Film

Dale Robinette/Dreamworks; Getty Images

"I had to be soft and curvy," says Chastain, who plays Celia in DreamWorks' adaptation of Kathryn Stockett's novel.

Jessica Chastain had some trouble gaining weight to play Marilyn Monroe-esque Celia in The Help.

"I had to be soft and curvy, so I ate a lot of soy," Chastain -- who is a vegan -- tells E! News.

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"Soy helps you become curvy because it has estrogen in it. So what I would do—and this is really gross—is buy cartons of soy ice scream, microwave it and drink it. It was disgusting… [But] wait till you see my boobs in the movie," she joked.

Chastain added about 15 pounds to her frame.

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"It was like a form of torture because you put on all this weight and then you're in the South where it's really hot and muggy and you're putting on girdles to suck you in," she said of the Jackson, Miss.-based movie, which opened to $5.5 million on Wednesday.

But she quickly dropped it after filming wrapped on DreamWorks' adaptation of Kathryn Stockett's bestselling novel.

"I do a lot of yoga," she says. "I'm also vegan, so that helps. I lose weight really quick."