In Bryan Stevenson’s 2014 memoir, “Just Mercy: A Story of Justice and Redemption,” the activist lawyer and viral TED Talker recounts his move to Alabama, his co-founding of the advocacy organization Equal Justice Initiative and his representation of Walter McMillian — a wrongfully convicted death-row inmate Stevenson worked to exonerate.

This December, the story will be brought to the screen by director Destin Daniel Cretton, with Michael B. Jordan as Stevenson, Jamie Foxx as McMillian, and Brie Larson as Eva Ansley, EJI’s other founder. As a film, “Just Mercy” is a courtroom drama, a character study and an indictment of the United States’ racist legal system.

Larson first learned of Stevenson and EJI from her friend and frequent collaborator Cretton. Over dinner in Montreal as they prepared to shoot “The Glass Castle” — their second film together after 2013’s “Short Term 12” — Larson and Cretton were “doing our usual,” she says. Their usual, according to Larson, is “talking about humanity, and what’s important to us.”

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Larson says Cretton told her, “Dude, you’ve gotta read this book ‘Just Mercy’ — it’s heartbreaking and hopeful at the same time, and I think that you’ll just really relate to what Bryan’s saying.”

She read it right away. “And it just gave me this fire inside,” she says.

When Cretton wanted Larson to play Eva, Jordan and Foxx were already cast. She asked Cretton whether they wanted her — a white woman — as their co-star in the movie. Yes, they wanted Larson, a best actress Oscar winner for “Room,” to join them.

“OK, then I’m good,” she says. “Because this is a form of ally-ship, and because both of them are the ones really doing the heavy lifting. These are extremely vulnerable performances.”

Larson talked extensively with Ansley. “This is not somebody who is a lawyer,” she says. “This is a mom who noticed something that was happening in her community, and nothing was going to stop her from rectifying the situation in whatever way she could.”

When the film was in production in Alabama, Ansley and Stevenson would visit the set, and Larson was impressed by the intimacy of their approach to activism. Ansley focuses on the “micro,” Larson says. “How can you be decent to every single person you encounter? It starts with you, and then it’s just in a one-foot radius around you.”

As for Stevenson, Larson says, “His belief is that the only way you can really figure out how to solve an issue is by getting really close to it.”

Stevenson and EJI will certainly receive more attention after “Just Mercy” comes out, which Larson hopes will result in more donations and help for incarcerated people. But she also wants the film to inspire audience introspection: “We as individual people need to look at ourselves and our biases, and where our hearts lie.”

Larson doesn’t know what the tangible effects of “Just Mercy” will be, but she’s taking a cue from Stevenson’s outlook. “I feel it’s only right that I do feel this sense of hope and optimism,” she says. “Which is just truly from spending so much time with Bryan.”