Wyatt Cooper, Anderson Cooper, Carter Cooper and Gloria Vanderbilt in "Nothing Left Unsaid"

“I’ve always though of my mom as coming from a time and place that don’t exist anymore,” muses Anderson Cooper, a little wistfully only minutes into “Nothing Left Unsaid,” a new film about his mother Gloria Vanderbilt. Then again, few ever breathe such rarefied air, which isn’t an allusion to the fortune Gloria inherited as a great-great granddaughter to the shipping baron Cornelius Vanderbilt, but to the incredibly rich life she has led, from being the center of a custody battle dubbed at the time, “the trial of the century,” when she was 9 to a succession of suitors as a young woman that included conductor Leopold Stowkowski, Frank Sinatra and Sidney Lumet, ultimately distinguishing herself as an independent force to be reckoned with as a canny fashion entrepreneur.

It is to the great credit of recently Oscar-nominated director Liz Garbus (“What Happened, Miss Simone?”) that “Nothing Left Unsaid” would seem to exist in such a space as well, flitting between the past and present and though few lives have been more well-documented – in addition to the press attention she received since birth, she’s written six memoirs of her own – the film reveals a side of the woman that has been little seen, including by her own son Anderson, who grills his 92-year-old mother with the same curiosity as any guest on “360°.” A lively conversation between the two is peppered throughout intimate home movies and eyefuls of Vanderbilt’s artwork – paintings and sculpture that help shed light on a life that proves the main that money can’t buy happiness, given the personal tragedies she’s endured over the years.

For Garbus, it’s another opportunity to demystify one of the great cultural figures of the past century, finding the art in what they did while creating some of her own and shortly before the film premieres on HBO following a well-received bow at Sundance, the director spoke about building the film around the dynamic talk between Vanderbilt and Cooper, her relatively recent turn towards biographies, and having too much good material.

Gloria Vanderbilt in "Nothing Left Unsaid"How did this come about?

Anderson [Cooper] had bumped into [HBO Documentary Films President] Sheila Nevins at an event for the late photographer Gordon Parks, who is another man that Gloria Vanderbilt had dated in her time, and he suggested, “Sheila, how about doing a documentary on my mom?” She went and visited with Anderson and saw that he had boxes and boxes of videotape that he’d been shooting over time, and then connected me with Anderson. It was Anderson’s brainchild, and I think it was his longing to have this conversation with his mother before it got too late to ask the questions, to learn about her life, which he really hadn’t known much about.

I think probably many of us don’t know that much about our parents’ lives or maybe even choose not to. He had a real desire to spend the time to focus on that, and then had all these video tapes of his times with her, so I thought it was really interesting. Gloria Vanderbilt was not who I expected her to be, and once I met her and saw their relationship, I felt there was a real opportunity in terms of how to tell the story and I was hooked.

There’s a central conversation between Anderson and Gloria that anchors the film. Was it always a foundational element of this?

It’s definitely a foundational element. I had been shooting Gloria in her studio making art, around her space, but the interview where they sat across from each other, it took place over the course of two days, and we gave Anderson 20 pages of questions. We read all the books that Gloria had written, and tried to understand the history as best we could and prepare him, because part of it was that he hadn’t done all of that himself. This was just the process of him doing that.

And yet this is a mother and son, so was there any concern the conversation might be too insidery for an outside audience to have interest in or did you know from the start that you could convey that they had this genuine curiosity about one another?

Yeah, but obviously there are some parents and children who are incredibly close, and in terms of family history, I do wonder how much we all really have had those conversations? I don’t know. I know my mother was married briefly before my father and I don’t know really much at all about it. There are ways that you assume you understand these things because you’ve grown up around them, but really when push comes to shove, you can’t really get too much detail about it and Anderson says it himself — he had spent so much time in the field, his life is so busy and he spends so much time abroad — that again, he hadn’t take the time to have those conversations.

Gloria Vanderbilt in "Nothing Left Unsaid"Was it interesting working with subjects who were so conscious of their image?

Yeah, because actually I didn’t feel it. Gloria talks about feeling more comfortable in makeup, which is in the film, and she really was very comfortable and unguarded with me, and with my crew. Anderson obviously lives such a public life, and he’s so generally used to being on camera — he was very much the same on camera as he is off — so it all flowed very organically.

You’ve actually said it was Gloria’s art was your way into this story. How did it open up the film to you?

I’m not so interested in making documentaries that have 15 talking heads, and everybody talking about a life. I’m interested in trying to find a way in that feels more organic and poetic to me, so when I saw the artwork, [I saw] Gloria was just constantly rehashing these moments of her past in both these very magical and very sad ways, so it just presented a really wonderful visual opportunity. Gloria Vanderbilt was nothing if not an aesthetic [thinker], so to use her own aesthetics as a way into her story felt right.

It seems like there’s been a shift in your career from films from “Girlhood” and “The Farm: Angola, USA,” which had a broader focus, typically on social justice issues, to these portraits like this and “What Happened, Miss Simone?” Has that been a conscious move, whether it’s because your interests have changed or it’s where the work is?

At this point, I’ve become conscious of it because there’s been a string of these, though I have another film that actually hasn’t come out yet which is much more in the verite style of the earlier films that you mentioned. When I first had kids, I had just finished “Girlhood,” and I made that film over the course of four years, and you get calls at 8 pm that really you should be there the next morning and it’s a way of life that’s not compatible with small children. So I said I wanted to do a film that was more structured where I could actually book a shoot a week in advance and not be running around like an emergency room call. That was when I made the film about Bobby Fischer [2011’s “Bobby Fischer Against the World”], and of course, I adored making that film. It’s still such a special film to me and it led to these other films, but my next film will not be a biographical picture.

Is it a different kind of filmmaking, dealing with archival footage as much as your subject may be in the present?

At the end of the day, the really heavy lifting of documentary filmmaking happens in the edit room, and it’s about crafting a story that’s moving, in some way adds to your life, and hopefully gives a little bit of insight into the human condition, which makes us more compassionate or understanding of others. Every film I’ve made has been about that a little bit, and that shaping process in the edit room is really the same, whether it’s “Girlhood” or Nina Simone, or Gloria Vanderbilt. They’re such different subjects, but it’s the same goal and challenge in the edit room.

Gloria has this great quote near the start of the film about “replaying scenes from her life and reorganizing them.” Did that quote actually influence the editing of the film in how it flips back and forth in time?

It really inspired us to think about how to visualize that in the editing. There were these incredible overlaps between past and present in the film, between art and archival footage that we found, and contemporary footage with Anderson. We took that opportunity to layer that material.

One of the other ways you’re able to bring the past into the present is with the wonderful animation that brings Gloria’s art to life. How did that come into the mix?

I worked with an incredibly talented animator named Molly Schwartz, and one of the first things I noticed in Gloria’s art studio was this dollhouse that she had created. It was all with a mother and a father, and a perfect living room with a Christmas tree — this fantasy that she kept on working on, so when she would tell some these stories of her life we thought that we would build these dollhouses and tell the story. All of those dollhouses were built by my animator, and hopefully demonstrated some of her fantasy life.

Gloria Vanderbilt and Anderson Cooper in "Nothing Left Unsaid"You actually show her storage room, which seems like the mother lode for a documentarian. What was it like going in there for the first time?

Oh God, it was the mother lode. The truth is I made a film about Marilyn Monroe [2012’s “Love, Marilyn”], and you would think that she’s the most photographed woman ever, but with Gloria Vanderbilt, because she was famous from the moment of birth, the amount of material — not just what they had in their storage rooms, but what was already existing in the world — was so massive, I just felt like I want to turn over every stone. It was such a large amount that it always made us feel anxious that maybe there was more hiding somewhere, and there probably is, but it was an incredible trove of material.

Was it difficult then to narrow it down for the story you wanted to tell? You mentioned Gordon Parks earlier, who is only briefly alluded to in the film.

This is always the thing with the people I’ve made films about, like Nina Simone. There’s always this question, which you’re not asking, but it’s “Why didn’t you talk about this?” And it’s always funny, because you’re like, “Well, you really want the film to be 17 hours long?” My thing is I don’t go in with a preconceived notion of this particular factoid of life has to be in there. I work with what I feel in the material best expresses the heart and the essence of the story, and so sometimes that means some anecdote about a Gordon Parks, or about the jeans business, if it’s less than tactful, it’s not going to be in the film.

“Nothing Left Unsaid: Gloria Vanderbilt and Anderson Cooper” premieres on HBO on April 9th.