Patricia Clarkson remembers exactly how she found out she was being underpaid as an actress.

“About five, six years in, I finally asked my agent, ‘What is my male co-star being paid?’” she told Page Six exclusively at a Cinema Society screening of her new movie, “Lilly,” earlier this week.

The Oscar nominee, 65, was shocked by his response.

“He said, ‘You don’t want to know,’” she remembered.

The “Easy A” star immediately sprung into action.

“I started having my agents question everything,” she explained. “I started to demand equal pay long ago. I would say, ‘Well, if [my male co-star’s] making this…We have similar-sized roles.’”

Clarkson remembered that producers would attempt to continue paying her less by claiming that the male co-star “has more credits and I would say, ‘No, that doesn’t fly. He doesn’t have more credits.’”

The “High Art” star said that her income increased by 20% once she began demanding equality.

Equal pay is at the heart of her new movie, “Lilly,” in which she plays Lilly Ledbetter, an American activist who fought tirelessly against pay discrimination after discovering she’d been underpaid for decades while working for an Alabama tire factory.

The case made its way to the Supreme Court, and the Lilly Ledbetter Fair Pay Act was the very first law that President Obama signed when he came into office.

Clarkson declined to meet Ledbetter, who died last year at age 86.

“I chose not to. I had her on such a high pedestal. I was raised in a family of five daughters with a mother who ran a city,” she said of her mother who was a New Orleans councilwoman and politician. “Lilly Ledbetter was God to us and I had to take her off the pedestal.”

The Tony nominee said that Ledbetter understood the decision and told her in a phone call, “We’re going to meet at the end.”

Clarkson hopes that the movie’s powerful message resonates with “every woman across the country.”

Hillary Clinton, Sarah Jessica Parker, Matthew Broderick, Katie Couric, Jim Parsons and John Slattery also attended the screening.