THE SON STAYS IN THE PICTURE: Filming the upcoming HBO documentary “Nothing Left Unsaid: Gloria Vanderbilt and Anderson Cooper,” Liz Garbus captured more than their fractured family history.

While Cooper is widely known for his CNN air time, Vanderbilt also appears to be a never-enough worker, based on clips of her in her art studio, where she can be found each day even at 92. Garbus spoke of their unexpected similarities.

“They are both very, very private people. One might have the expectation that Gloria is [going] out to lunch every day for the past 10 years or is going out to parties. While she does occasionally, her life is really much more solitary and it’s about her work. I think you could say the same for Anderson. Obviously, his career is very public, but his private life is very private. I think they both need that time in their heads that their lives can allow them.”

Cooper, who lost his father Wyatt and his brother Carter by the time he was 21, says in the film, “Some people are sucked under by tragedy and loss, and it destroys them. And some people, it propels them forward. It certainly has for my mom and it certainly has for me.”

Cooper chalked up his own full-speed-ahead way of living to something he gleaned from a Jacques Cousteau special about sharks around the age of 11. “If sharks stop moving forward they die, because they can’t breathe without forcing water through their gills by the forward motion,” he says. “That notion has always really resonated with me — the need to continue to move forward and to continue to breathe.”

The same might be said of Vanderbilt, who is filmed several times discussing her paintings and art. “When I first met her and walked into her studio, I thought, ‘Ah, this is how you want to tell this story. You want to use the artwork as a flashlight kind of into her past history,’” Garbus said. “As a businesswoman, she was incredibly successful. Look, she wasn’t bred as a Vanderbilt young woman to be a career woman. But she did have some strong female influences, her aunt Gertrude Whitney being one of them. Gloria found a path forward and clearly work is a big part of her identity and what sustains her.”

Cooper also managed to keep himself on the clock, so to speak, volunteering to handle the sit-down interviews with his mother (which both preferred to do barefoot) and are featured throughout the 107-minute film that makes its debut April 9. “At first, I was like, ‘Oh really, because I do those.’ But I realized what was key in this film and what made it sort of special and unique was the dynamic between the two of them,” Garbus said. “That was kind of a more interesting negotiation, because in some ways I’m not used to working with somebody who has some knowledge of how the sausage is made. So it was about balancing creative freedom while being respectful and inclusive.”

Garbus declined to comment about reports she will make her narrative feature directorial debut with Amazon Studios’ film adaptation of “Lost Girls,” based on the Robert Kolker 2013 book about a serial killer targeting sex workers on Long Island. (Garbus and her collaborator Rory Kennedy are two-time Oscar nominees.)

Having directed and coproduced last year’s documentary about Nina Simone, “What Happened Miss Simone?” Garbus only skated around the edges of the controversy surrounding the upcoming Simone biopic starring Zoe Saldana. “What I’ll say is people are very upset including the family members, not just because of the casting, but also because the story didn’t feel like a true story to those who knew Nina Simone very well,” she said. “So I think it’s very complicated. But I haven’t seen the movie and in fairness to the director I wouldn’t want to comment on the family’s opinion on the movie.”