Jessica Chastain recalls fighting back after her mother's boyfriend slapped her

Jessica Chastain promises "major changes" were coming to Hollywood as she accepted the chairman's award for "Molly's Game" at the 29th annual Palm Springs International Film Festival on Tuesday, Jan. 2, 2018, in Palm Springs, Calif. (AP)

Jessica Chastain will not be bullied.

The 40-year-old actress, along with Michael B. Jordan, covers WSJ magazine's Talents and Legends February issue, and recalls having to physically stand up for herself as a child.

After disclosing that she grew up with a single mom, and to this day does not know who her biological father is, Chastain shared a "turning point" in her life when she had an encounter with her mother's then-boyfriend.

"He did something -- my room was messy or whatever and he had taken my clothes, and I was telling him to give me back my stuff -- and he slapped me. And I just kicked him in the genitals, and he fell to the ground immediately," she revealed. "It was me, my sister and my brother -- and I remember looking at my sister’s face, and we were both like, ‘Oh, my God, what did I just do?’ And then I ran out of the house."

Chastain added, "I always look back on that moment as knowing that, 'OK, if anything happens to me, I’m capable of fighting back.' He never messed with me again."

The two-time Oscar nominee also offers up some advice for those that are being mistreated.

"If you allow a bully to intimidate or victimize you, they’ll continue to do it," she said. "Bullies are actually weak; they don’t go after strong people."

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Chastain seems to practice this point of view in her professional life, and notes the time former movie mogul Harvey Weinstein -- who has been accused of sexual misconduct by numerous women -- introduced her at the premiere of "The Disappearance of Eleanor Rigby," which was distributed by the Weinstein Company.

"He actually told the audience, ‘If I had to get in a boxing ring with Muhammad Ali or Jessica Chastain, I would choose Muhammad Ali," she recalled.

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It's been a good year for Chastain. In addition to gaining critical acclaim for her lead role in Aaron Sorkin's "Molly's Game," she also married Gian Luca Passi de Preposulo in June.

"I never wanted to get married,” she admitted. “When I first met my husband, he knew that marriage wasn’t something I was interested in, and then, as we got to know each other, the idea of marriage shifted for me. There are some things worth celebrating -- and he’s worth celebrating."

Further gushing over her home life, Chastain continued, "I actually love being married. I never thought I would, but this is a spectacular human being, and I am celebrating that I get to share my life with him."

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In November, the A-list actress spoke to ET about harassment in Hollywood.

"I want to be a part of an industry that is very inclusive," she proclaimed. "One that teaches empathy, and the only way you do that is, you learn about someone who doesn't look like you. And I think there's a generation of artists that that is their goal."