PLUS: Carrie Fisher opens up about the pressure to lose weight for Star Wars: The Force Awakens
Credit: Ian Gavan/Getty

Carrie Fisher, who debuted her mother-daughter documentary Bright Lights: Starring Carrie Fisher and Debbie Reynolds in Cannes this week, said her mom powered through in the face of health concerns.

Fisher, 59, and the documentary’s director Fisher Stevens sat down with PEOPLE and Entertainment Weekly for a Facebook Live interview in the French city this week, to discuss the film – and how Reynold’s triumphed through health issues to be part of the project.

“It’s a lot of times terrifying, but watching my mother, who’s incredibly resilient, coping with certain health issues that she’s had,” Fisher said of her 84-year-old mother. “We were really lucky we got really what probably could be her last [big project]”

The documentary, which premiered at the 2016 Cannes Film Festival earlier this week, is an intimate portrait of Fisher and Reynolds – both Hollywood royalty – throughout their decades-long careers.

Stevens said that the Singin’ in the Rain star initially didn’t understand the idea of doing a documentary – “she asked for a script when we started shooting” – but began warming up to the venture, even amid health issues.

“There’s a moment in the film where she calls [ co-director Alexis Bloom] and says, ‘I can’t film today,’ ” Stevens told PEOPLE and EW. “There were moments where she did cancel but we were able to capture, I think, sort of, lightening in a bottle.”

Fisher said that her mother is “doing really well” although she’s “been a little more frail.”

“She had an illness that she’s recovered amazingly from,” Fisher said , noting that Reynolds “had a spinal issue.”

The 59-year-old told the Washington Post that she wanted to make the film because Reynolds had begun to decline both cognitively and physically.

“I didn’t know how much longer she would be performing,” Fisher said of her mother. “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.”

Despite her close-knit relationship with her mother, and working so closely with her on the documentary, Fisher said there is one thing she couldn’t quite open up about: Star Wars: The Force Awakens

Fisher reprised her role as Princess Leia in the highly anticipated film, and Stevens said her filming the movie overlapped shooting the documentary.

“Carrie had to keep very quiet about it,” Stevens told PEOPLE and EW, noting that he went to visit Fisher while she shot Star Wars in London but wasn’t allowed on set.

Fisher said that, in the documentary, Reynolds doesn’t learn anything about Star Wars – “If she does she would never tell.”

She told PEOPLE and EW about the pressure she was under to lose more than 35 lbs for the movie.

“For a lot of people it’s infuriating that I actually aged I didn’t realize that [they] had written a contract that said I would stay looking the exact same way – 23,” she said.

“I don’t like that part of the business but, you know, I like being in Star Wars. And it’s hard for me to look at myself at any age. So they did the best they could with what they had.”