"We were meant to work together," Kidman reminded Stewart. "... I remember [director] David Fincher saying, 'Oh my god, we have discovered the most incredible actress.' And then I got injured and ended up not playing your mama."
The role of Stewart's mom ultimately went to Jodie Foster, but Stewart still looks back fondly on the brief time she spent working on the project with Kidman.
"We spent a couple weeks rehearsing. I have a vivid memory of it, because the way you treat kids is so telling. I felt like I was buds with you," Stewart recalled. "It was two or three weeks, but I was always like, 'She’s one of my friends.' You gave me walkie-talkies for Christmas."
"Here we are decades later, hanging in there," Kidman replied. "And you’re amazing; Fincher was right, as he always is."
The women are certainly thriving in their careers, having both taken on the challenge of playing real people -- Princess Diana for Stewart and Lucille Ball for Kidman -- in their latest projects.
"This was not something that I overthought, because as soon as you start picking this one apart, it’s easy to find reasons to not do it," Stewart explained of taking on the role in Spencer. "... If you’re going to be an actor still, you should do something like this -- or just stop doing it."
Playing Ball in Being the Ricardos was an easy choice for Kidman, but sticking with it in the face of fear was another matter entirely.
"I knew Lucy and the I Love Lucy show, but when I read the script, I had no idea of her story and her journey, and I related to parts of it," Kidman said. "I was really game to give it a go. And then the flood of fear, I suppose, came in and I went, 'Oh no, what have I done?' Which I do a lot in my life, and then I push on through."
When ET spoke with Kidman earlier this month, the actress reflected on the "incredible passion" she still has for her career. Watch the video below for more of Kidman's interview with ET.