"I remember, as a kid, watching old movies with my mom and turning to her and [asking], 'Did you ever know Errol Flynn?' And she'd be, like, 'Oh, yes,' " Cooper said. "And that would be it. She wouldn't say anything more. But I always knew there was a lot more there."
And it turns out, she dated Flynn when she was 17 and he was 32.
"She's had a much more interesting life than me," Cooper said.
But, like most mothers, Vanderbilt never really opened up to her son. Until he started going through boxes and boxes of things she stored away for years. Decades.
Cooper called up images of the scene in "Citizen Kane" when the title character is in a storage room filled with boxes. Or the final scene in "Raiders of the Lost Ark."
"One box you would open up and it would be a box of amazing letters from Howard Hughes, who my mom dated when he was hot Howard Hughes,'' Cooper said. "And then you'd open up another box and it would be a box of Cornflakes from 1953 that somehow, in a move, got packed away and my mom has been paying to store all these years."
"This sounds like that show 'Hoarders,' " Vanderbilt said. "It wasn't really quite like that."
Cooper and Vanderbilt turned her possessions and dozens of interviews Cooper did with his mother over to filmmaker Liz Garbus, who crafted "Nothing Left Unsaid: Gloria Vanderbilt & Anderson Cooper," which premieres Saturday at the Sundance Film Festival in Park City.
"Her entire life has played out on a very brightly lit stage," said Cooper. But the idea behind the documentary is, "You may know her name, but you really don't know who she is or what her story is."
Vanderbilt has written four autobiographies, but never reads anything anyone else writes about her — a decision she made after her aunt won the custody battle.
"I see my name in the paper, I just turn the page," Vanderbilt said. "Because I wanted to become a person. I wanted to make my own life without a lot of baggage hanging over me."
She cooperated fully in "Nothing Left Unsaid." She did hours of additional interviews with her son, which he described as "kind of surreal."
"I think that's kind of one of the messages of this film. This idea of leaving nothing left unsaid is something that I feel really strongly about," Cooper said. "And I didn't want to have that feeling with my mom.
Cooper's father died when he was 10; Vanderbilt's died when she was 19 months old. And they share "this fantasy that there was a letter out there somewhere" from their fathers, Cooper said. "And both of us still kind of secretly believe that letter will someday show up. "
"It will," Vanderbilt insisted. "It's still going to show up. I absolutely guarantee it."