Anderson Cooper, Gloria Vanderbilt Joke About Her Famous Dating History

Anderson Cooper - H 2015
AP Images/Invision

Anderson Cooper - H 2015

Anderson Cooper keeps a very tight schedule.

Just an hour before his Thursday interview with President Barack Obama for CNN's live "Guns in America" town hall, the 48-year-old journalist appeared, via satellite, to speak with mother Gloria Vanderbilt about their upcoming HBO documentary.

Nothing Left Unsaid finds Cooper sitting down with his 91-year-old mother, exploring intimate details of her life — one that's vacillated famously between tragedy and glamour. "I think my mom has been in the public eye for longer than anyone else alive," explained Cooper, speaking with Vanderbilt and filmmaker Liz Garbus — both of whom appeared in person at the Television Critics Association winter press tour. "Her entire life has played out on a very brightly lit stage."

The pair charmed with mother-son banter, despite the fact that they were separated by more than 2,000 miles, but it was the end of the conversation that got the strongest reaction. When HBO Documentary Films president Sheila Nevins announced there was time for one last question, a reporter asked Vanderbilt if she really did date actor Errol Flynn.

"I did date Errol Flynn," she said, pausing a moment, "once."

To the heiress' right and left, two big screens showed Cooper repeat the word "once" while making air quotes.

"Tell them about your date with Marlon Brando," he added, prompting some rarely visible glee from the room of reporters.

Nothing Left Unsaid premieres at Sundance before it bows on HBO in April.