Oprah Winfrey

NEW YORK — Oprah Winfrey doesn’t scare easy and she wasn’t frightened here.

“But I was unsure and uncertain of myself going into this role,” she says. “I did not want to do it. I never truly expected to do it.”

Instead, it’s Winfrey who erupts in the new HBO film “The Immortal Life of Henrietta Lacks” as a tormented woman in search of the mother she never knew whose tissue sample would yield medical marvels benefiting millions.

The film, which premieres Saturday at 8 p.m., is based on the best-seller by Rebecca Skloot. It charts the rocky road to discovery shared by Henrietta Lacks’ daughter Deborah (Winfrey) with Skloot, who wanted to shine light on the human story behind the legendary cell line known as “HeLa.” Rose Byrne (“Damages,” “Bridesmaids”) plays the intrepid reporter Skloot.

Winfrey was captivated by the book and acquired the rights with the intent of producing a film. Then two things happened to set the project on its proper course.

She heard one of the hundreds of interviews Skloot had made with Deborah Lacks (who had died just months before the book’s 2010 publication). Winfrey heard her on tape saying to Skloot, “Girl! Did you see ‘The Oprah Show’ today? SHE should play me!”

“I did it as a way of honoring her,” Winfrey says, “honoring the legacy she tried to create and build for her mother.”

The other reason Winfrey couldn’t say no to the role: George C. Wolfe, the celebrated Tony Award-winning director, joined the project. Wolfe saw the film as more than an untold tale of science.

“The desire to know one’s parents — that’s a very primal thing,” he says. “They are literally and metaphorically the DNA of who we become. For Deborah to know her mother is to know her own story. That’s the driving energy on which everything else in the film can hang.”

Even the simplest things Deborah wants to know: “Did she breast-feed me? Did she love to dance?”

A poor tobacco farmer who worked the same Virginia land as her slave ancestors, Henrietta Lacks died in 1951 at age 31.

“In segregated America, on paper, she had no power,” says Wolfe. “But her HeLa cells were unbelievably powerful. That juxtaposition was really fascinating to me.”

The film was shot last summer in the Atlanta area, plus a few days on location at Johns Hopkins Hospital in Baltimore.

Byrne reports that during the production, “I didn’t see the Oprah that we all know: ‘OP-rah WIN-frey!!!’ She was very focused, very meditative.”