"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."
"Nothing Left Unsaid" is a portrait of a happy, optimistic woman who is "far more joyous than me," Cooper said. "She does have this incredibly optimistic way of looking at things. She really understands the next great love is right around the corner."
"Absolutely," Vanderbilt interjected.
"And [she believes] there is some guy on a boat in the Mediterranean just waiting to whisk her away," Cooper said. "And there very well may be. Nothing would surprise me with my mom."
Both said that, at ages 91 and 48, they're closer than ever.
"I do think nothing has been left unsaid. I know my mom in a way that I didn't know her a year and a half ago, when we started this process," Cooper said.
"We started communicating and became close in a way that we hadn't before," Vanderbilt said. "I feel that I understand you more and love you more, if that's possible."
And the documentary is not just about a rich and famous mother and son it's about a mother and son.
"As a witness to this process, I think that it's made me a better daughter to my mother," Garbus said. " I think there's something very large that we can all take away from this. And that's a very profound gift that Anderson and Gloria have given us."
"Nothing Left Unsaid: Gloria Vanderbilt and Anderson Cooper"
Premieres • Saturday, Jan. 23, at 5:15 p.m. at The MARC in Park City. Q&A with Cooper and filmmaker Liz Garbus follows.
Other screenings • Sunday, Jan. 24, noon, Grand Theatre, Salt Lake City; Monday, Jan. 25, 12:30 p.m., Redstone Cinema 1, Park City; Wednesday, Jan. 27, 3 p.m., Sundance Mountain Resort screening room; Saturday, Jan. 30, 8:30 p.m., Prospector Square Theatre, Park City. Joint memoir by Vanderbilt and Cooper coming April 5
A joint memoir by Anderson Cooper and Gloria Vanderbilt will be published in April, just days before the airing of an HBO documentary featuring the famous designer-socialite and her CNN anchor-son.
Harper, an imprint of HarperCollins Publishers, said it had acquired "The Rainbow Comes and Goes: A Mother and Son Talk About Life, Love, and Loss." The book will be released April 5, four days before HBO airs "Nothing Left Unsaid," which also screens this week at the Sundance Film Festival. CNN will show the movie on April 30.
According to Harper, the book will feature "hilarious and always touching exchanges" between Cooper and Vanderbilt and will contrast Vanderbilt's "unwavering optimism" with her son's "darker outlook."