Fisher, whose mother-daughter documentary "Bright Lights: Starring Carrie Fisher and Debbie Reynolds" premiered in Cannes, France, this week, opened up about Reynolds' health during a recent Facebook live interview with People and Entertainment Weekly.
"It's a lot of times terrifying watching my mother, who's incredibly resilient, coping with certain health issues that she's had," Fisher, 59, said of her 84-year-old mother. "We were really lucky...we got really what probably could be her last [big project]."
Although Reynolds is "doing really well," Fisher said, she's "been a little more frail."
She said her mom "had an illness that she's recovered amazingly from," while noting that Reynolds "had a spinal issue."
Director Fisher Steven, who was also interviewed, said Reynolds didn't understand the concept of a documentary at first.
"She asked for a script when we started shooting," Stevens told People and EW.
She eventually warmed up to the idea and persevered despite her health issues.
"There's a moment in the film where she calls [co-director Alexis Bloom] and says, 'I can't film today,'" Stevens said. "There were moments where she did cancel but we were able to capture, I think, sort of, lightning in a bottle."
The film, an intimate portrait of fame, family, aging and mother-daughter dynamics, has received enthusiastic reviews in Cannes and will appear on HBO early next year.
Earlier this week, Fisher told the Washington Post that she wanted to make the film because Reynolds, who still occasionally performs her nightclub act, had begun to decline both cognitively and physically.
"I didn't know how much longer she would be performing," the "Star Wars" star said of her mom. "It's the thing that gives her life, but it was also pulling it out of her, because she'd perform and then she'd have to recover. But this is someone who wants to go back and do it now."