One day, circa 2000, Rebecca Skloot was riding in a car with Deborah Lacks, whose late mother, Henrietta, had posthumously and unwittingly contributed to some of the most important medical research of the 20th century.

At the time, Skloot was a scrappy but barely published young journalist, and it would take a decade for her to complete the nonfiction book “The Immortal Life of Henrietta Lacks.” But Lacks had a prescient vision:

“‘This book is going to come out and it’s going to be a bestseller. There’s going to be a movie. Oprah is going to star in it,’” Skloot recalls Lacks saying during that drive. “And I was there going, ‘Deborah, Deborah, whoa, whoa, that’s crazy. Let’s be realistic here.’”

Nearly two decades later, Lacks’ prediction — which Skloot had forgotten until she recently unearthed it on an old audiotape — has been realized. Not only did Skloot’s 2010 book become a runaway bestseller, but it has been adapted into a movie starring, yes, Oprah Winfrey.

Written and directed by George C. Wolfe and premiering at 10 p.m. Saturday on HBO, the film follows Skloot (Rose Byrne) and Deborah Lacks (Winfrey) on a journey to understand more about Henrietta, who died of cervical cancer at age 31 in 1951, leaving behind five small children and a legacy that would change modern medicine.

Unbeknown to Henrietta (played in flashbacks by Renee Elise Goldsberry), a doctor at Johns Hopkins University in Baltimore removed cancer cells from her cervix. These cells were able to reproduce outside the body at astonishing rates, making them ideal for medical research. The mass-produced HeLa cells, as they became known, have contributed to major breakthroughs, including the polio vaccine and in-vitro fertilization — all thanks to a black woman who died in obscurity and whose family has never been compensated for her contributions to medicine.

In a call from her home in Santa Barbara, Winfrey recalls her initial impressions of Skloot’s book.

“Being a student of African American history, I was thrown by the idea that I’d never heard of Henrietta Lacks, and especially having lived in Baltimore, and traveled the streets she lived on and been a reporter in that town for eight years and never having heard her name — that’s what was fascinating to me,” says Winfrey.

Shortly after the book’s publication, Winfrey teamed with HBO and “Six Feet Under” producer Alan Ball to adapt the hotly contested property. “One of the defining characteristics of my personality is whenever I discover anything, whether it’s a great juicer, or as a kid a great candy bar, or as now a great story, I love to share it,” she says.

Skloot, who spent a decade reporting and writing the book and forging close relationships with the Lacks family, was understandably protective of the material. But she’d been impressed by HBO films such as “Temple Grandin,” about the autistic animal behavior expert, and “You Don’t Know Jack,” about euthanasia activist Dr. Jack Kevorkian — “movies that handled complex science ethics stories, where there’s a real gray area.”

The film has followed a lengthy gestation process, thanks in part of the challenge of adapting Skloot’s 400-page book — which deftly interweaves a painful family saga with complex science, racial history and medical ethics — into a concise screenplay.

Wolfe, a Tony-winning playwright and director whose theater credits include “The Normal Heart,” “Lucky Guy” and last year’s acclaimed musical “Shuffle Along,” made the decision to focus on Deborah’s quest to know more about the mother she lost as a baby.

Wolfe was particularly moved by the fact that Deborah had virtually no memories of her mother, though her cells were now “immortal” and had improved the lives of countless millions.

“Her desire to process this information, to gather pieces no matter how desperate, no matter how clueless, no matter how, sometimes, foolish it was, just to know — I found that heroic and thrilling,” Wolfe says during a break from editing in the frenzied final weeks before the film’s release. By design, the audience knows only as much about Henrietta as Deborah does.

As much as he revered Skloot’s writing, Wolfe was more interested in “embodying the world” than treating the book with kid gloves. “You honor the work by digging, you don’t honor the work by being passive to the process. You’ve got to get in there and wrestle. And if it’s a strong muscle, it can withstand your wrestling.”

Wolfe’s involvement also helped convince Winfrey, who hadn’t planned to star in the film but reconsidered, thanks to a recommendation from Audra McDonald. The “Shuffle Along” star urged Winfrey to work with Wolfe if she ever had the chance.

For the talk show doyenne and media tycoon, acting has been a successful if occasional side gig, with roles in films including “The Color Purple” and “Beloved” as well as a more recent recurring part in the OWN family drama “Greenleaf.” But performing feeds her in a way that’s different from, say, interviewing former First Lady Michelle Obama.

“It’s really stimulating to probe the depths of yourself, searching for emotions that otherwise would go untouched or unexplored, particularly for me,” says Winfrey. Still, playing Deborah, a devoutly religious but troubled woman with a host of eccentricities — in one scene she douses a restaurant table in Lysol before sitting down — was a challenge.

“I have to go really, really, really deep to pull up rage and anger. Really. Really. Really deep,” she says, enunciating in a way that can only be described as Winfrey-esque. “I don’t live in that space.”