"If you liked her, you're gonna love her" after seeing the film, O'Neal told AP.
"Farrah's Story" will show her "incredible strength," Stewart told AP. "Her big message to people is don't give up, no matter what they say to you, keep fighting."
It's a message that colorectal cancer specialist Dr. James Church from the Cleveland Clinic said applies to any kind of cancer patient.
"She helps a lot of people, not just with anal cancer, but other painful cancers," he said. "When they see someone who has the courage to really fight, that encourages them and their families."
But because Fawcett's cancer is highly unusual and details about its cause are unknown, Church said her impact on anal cancer so far has been minimal.
He said the clinic might see 400 to 500 cases of colorectal cancer in a year, and only 12 to 15 anal cancers.
Where he does see anal cancers on the rise is in the male homosexual population that engages in anal sex. In many of those cases, the human papillomavirus, or HPV, the most common sexually transmitted infection, is the cause of the cancer.
"I don't see Farrah Fawcett as the spokesperson for that," Church said about the growing numbers of gay men affected by anal cancer.
But if HPV is to blame in Fawcett's cancer, her case could serve as reminder to young women about the importance of the new vaccine against HPV.
Since Fawcett's cancer spread to her liver in February, the former pinup girl known for her golden tresses has lost her once-defining trait.
In tonight's film, O'Neal revealed that during the first rounds of treatment, they did all they could to preserve her iconic blond locks. But in February, when Fawcett resorted to chemotherapy and her hair began to fall out, she finally cut it.
Earlier this week, O'Neal told People magazine that Fawcett's condition has deteriorated.
"She's in bed with an IV. They're not trying any great measures to save her," O'Neal said.
Reports surfaced that the always slender Fawcett had shrunk to 84 pounds. It's a claim her doctor refuted on "Good Morning America" in April.
"She is 101 pounds. She has challenges every day with the fight with cancer. As long as she is able to fight, she will keep fighting," said Dr. Lawrence Piro.
Piro said Fawcett had a "reasonable weight" under the circumstances.
Fawcett is in a "very rocky place," O'Neal said. "We put on a brave front, always, when we're with her. She doesn't know how scared we are."
That includes Fawcett and O'Neal's son Redmond O'Neal, 24, who has just been admitted into a court-order drug program.
Recently, Redmond O'Neal, who has struggled with drug-related legal charges, was allowed to leave jail, where he was being held on a probation violation, to visit his mother.
"He crawled into bed with her in his jail jumpsuit and his shackles on, and he cuddled with his mother and cried," Ryan O'Neal told People magazine.
O'Neal told his son not to rattle his shackles and to hide them from his mother.