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. I had other people in mind 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. EDT, 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 stage and film 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, finding her way, like we all did.

“It was intimidating for me,” Byrne adds. “But that was good because that’s what Rebecca was: intimidated to try to tell this story (about Henrietta Lacks and her cell line) that she had been obsessed with since she was 15.”

The close but stormy relationship forged between Deborah and Rebecca is portrayed robustly by Winfrey and Byrne.

“The way you achieve that is by finding two people who are extraordinarily generous with each other,” says Wolfe. “Where one pushes, the other is there to receive the push and then push back. You can’t achieve that kind of connectedness with people who have their guards up.”

As for Winfrey in particular, Wolfe hails her as “brave and ferocious and willing.”

“I don’t have a lot of acting experience,” insists Winfrey, who says she learned her greatest acting lesson long ago, during her first, Oscar-nominated film appearance in the 1985 drama “The Color Purple.”

The director, Steven Spielberg, warned her that she would need to cry in a scene the next day. She feared she didn’t know how. She was frantic. Then a veteran co-star, Adolph Caesar, gave her wise counsel: “He says, ‘You got to let the character take control. And if SHE wants to cry, she will cry. But if SHE doesn’t want to cry, not even Steven Spielberg can make her.’ So giving yourself over is part of the process.”

Perhaps by now, at 63, Winfrey has learned to give herself over to the process in ways even beyond a film role: She says she’s easing up after all those hard-driving decades seeking more and more mountains to climb.

“The 60s are no longer about the climb. They’re about enjoying the view, the view that you created based on the long climb,” she explains. “I feel no need to prove anything anymore. The joy is in doing it, when you can come away from an experience savoring the view.”