“Hello, Bill, it’s Lord Sisley,” says a man in a posh English accent. “I’m in Africa. And dear God, man, you should be here. There’s creatures who would adore you. I send you all my love. But not like that day in boys school. Something different. Something wonderful. A hug. But if you wish, Bubbly, call.”

This is one of the many voicemails Robin Williams left for his longtime pal and fellow comedian Billy Crystal over the course of a decadeslong friendship. Like Lord Sisley’s communiqué, each message Williams concocted featured a new character formed inside his ever-churning brain ― a hobby taken up just to make his buddy laugh.

As Crystal explains in the new HBO documentary on the late, great actor, “Robin Williams: Come Inside My Mind,” he always knew that if he had a missed call from Williams it was going to be a wonderful day.

“The phone would ring and I’d look at it and see the 415 area code,” Crystal recalls with a smile. “I knew it was him. I knew it was going to be something really good.”

One time, Williams aped Ronald Reagan. Another, Sam from the Sibilance Society. So, when Williams underwent heart surgery in 2009, Crystal returned the favor by leaving him a dozen voicemails from Vinny the Valve Guy ― a mechanic character who “supplied the valve” for Williams.

“A day and a half [after his procedure],” Crystal says, “and he was in pain and had just gone through this massive surgery, he called me, ‘Oh god, ohhh, hilarious! You know, can I talk to Vinny?’”

“As the friendship really grew and grew, we kind of needed each other more.”

Actors Billy Crystal (left) and Robin Williams at the premiere of Magnolia Pictures’ “World’s Greatest Dad” on Aug. 13, 2009, in Los Angeles.

KEVIN WINTER VIA GETTY IMAGES

“Come Inside My Mind” director Marina Zenovich told HuffPost that she heard about the voicemails from an assistant of Williams’ longtime manager, David Steinberg, who was helping her and producer Alex Gibney secure old footage and audio of the comedian.

“So I chased down Billy to get the messages and it took awhile, but one day they appeared on my phone,” Zenovich said, her initial excitement still oozing through the phone. “It’s like, ‘Are you kidding me?!’ They were just so wonderful and showed the fun they had.”

“I think that they really loved each other and could keep up with each other,” she added. “One line, in particular, I love in the movie is when Billy says, ‘Everybody wanted something from him. I just liked him.’ You really got a sense of that.”

Zenovich, who interviewed everyone from David Letterman and Steve Martin to Williams’ first wife, Valerie Velardi, and his son Zak Williams, said there was something particularly moving about Crystal’s sit-down.

“I didn’t know what to expect with Billy and, you know, I was kind of nervous,” said Zenovich, who’s known for her documentaries on Roman Polanski (“Wanted and Desired”) and the Duke lacrosse scandal (“Fantastic Lies”). “But it was a very emotional interview because he just had such a heavy heart. I was just really moved by him talking about his friend. You really got the sense that he thought they would grow old together and they weren’t going to. And you could feel that sense of loss. That was just very intense.”

On the whole, her documentary centers around memories from the people who knew Williams best, along with anecdotes from the man himself. “Come Inside My Mind” is almost fully narrated by the Oscar-winning actor; his sketches and candid interviews with celebrity interviewer Lawrence Grobel form a throughline in the film. It’s powerful, funny and, well, extremely sad.

It’s a bittersweet experience for the audience, and for Zenovich, who recounted her time making a film about everyone’s favorite comedic genie in the conversation excerpted below:

Billy Crystal and Marina Zenovich attend the Los Angeles premiere of “Robin Williams: Come Inside My Mind” from HBO on June 27.

JEFF KRAVITZ VIA GETTY IMAGES

Why Robin, and why this portrait of his life?

I think for every documentary filmmaker, you’re always trying to figure out what to do next, what interests you, what intrigues you. I’ve been a fan of Robin since I was a kid. There are particular movies that spoke to me. I never saw him live, but he’s just utterly intriguing. His mind was just something that was a gift from God. So basically I decided to make a film about him, and there were two competing projects and we merged. And here we are!

What were these competing projects?

Alex Gibney was making a film and I was making a film. Alex is an old friend and a mentor, and so it was kind of one of those, “Are you making a film about Robin Williams? I’m making a film about Robin Williams!” So we decided to merge and he was the producer and I was the director.

Alex is someone who I’ve gone to for advice through the years. Documentary filmmakers are all kind of a similar breed ― we try to help each other.

Do you find that there’s a lot of overlap, especially in the documentary world, when you want to be current and cover topics that will interest viewers at a particular time?

Yeah, it’s hard because when things are happening, you’re kind of like, “Oh my God. I should be making a documentary about this!” Or it’s either you need to do it super fast or you need to wait awhile, you know?

So when you and Alex came together, at what point were you at in the process?

I was at the beginning and I think Alex was not even at the beginning, so it was perfect timing. I don’t have a big production company or anything so it was hard at first. It was during a time when the Williams camp wasn’t interested in doing anything, so it took some time. These things always do.

“His mind was just something that was a gift from God.”

– Marina Zenovich on Robin Williams

I’m sure you reached out to his widow, Susan Schneider, and his second wife, Marsha Garces, who weren’t in the film. How did that feel when they said, “I don’t want to be a part of it”?

Yeah. There was a time when my specialty was making films about people who weren’t in them. And so I’m kind of relentless for chasing people down and trying to get them to be in the film. I did that with [Williams’ close friend] Bobcat [Goldthwait] ― he really didn’t want to be in it and I respected that. With Marsha and Susan, I reached out to both of them. Both of them didn’t want to be in it. Neither did the two younger children, and it’s a little different in that I really respect that. I respect their need for privacy and not wanting to talk about it.

It’s hard, but you get over it. You move on and you try to be as true to the story as you can and pick archive and pictures that help you tell their story without their cooperation.

You ended up interviewing a lot of incredible people from Robin’s life.

When I reached out to Robin’s first wife, Valerie Velardi, she put me in touch with people. First, you reach out to people and either they’re OK with you or they’re not. If they are and they like you, then they connect you with other people.

Robin Williams performs stand-up comedy during a fundraiser to benefit John Kerry’s Senate campaign at the Wang Center in Boston on Oct. 23, 1990.

BOSTON GLOBE VIA GETTY IMAGES

You really shine a light on Robin’s personal life through interviews with his close friends like Eric Idle and Billy Crystal, who shares those great voicemails.

One of the great things about doing the film with the estate was that Robin’s manager was able to help us get some interviews with David Letterman and Billy Crystal. … You want these people to kind of be there to tell you about what it was like in the ’70s when they were all starting out. David Letterman talks about that and he says, “It was the best time of my life.” And you really feel like you’re capturing something that people love, you know?

And getting Zak, his son, was invaluable, because he could speak to being the child of Robin Williams. If we didn’t have him, we wouldn’t have that. He’s so thoughtful and well-spoken and really seemed to understand his dad so much. I was really pleased that he was willing to talk to us.

The archival stuff you used of Bobcat, for example, helps viewers understand what really was going on with Robin’s diagnosis of Lewy body disease.

Oh, totally. I mean, it’s like who was going to say that? We didn’t have Robin’s widow saying it or Robin’s son, so I thought Bobcat would be perfect, because he talked about it in an interview and he was close to him. So you make these choices as you go along and sometimes when you don’t get interviews, you end up finding a way to make it work.

I love that the film focuses on Robin and his career and not too much on his death. Is that something you were aiming for, to give people an insight into his life more so than his death?

We knew from the beginning we didn’t want to focus on the death. We knew that it was going to be a part of the story, but I just felt he had such an amazing life and there was so much energy to it. We knew we always had to tackle his death and didn’t know how we were going to. But when I interviewed people I didn’t even have to bring it up, because they would bring it up.

I feel like for my characters in the Robin film, they wanted to show up for Robin and they wanted to tell their stories. All these little moments paint a portrait of a life that you’re trying to capture without the main character. You have his voice and you have his image and you have him doing stand-up and acting, but it all helps.

Just to go through all the photos and clips and get permission to use that sort of content must have been tough. Can you talk about the process of gathering the information and piecing it together?

I came up with an analogy of spinning a lot of plates. What was great with partnering up with Alex is he had the big production company, we had a big research team. They were in New York, I was in Los Angeles. My other producer Shirel Kozak was in New York. We were working with the estate, I was working a lot with Robin Williams’ former manager David Steinberg, who was helping us ― just gathering footage, looking for archives, reaching out to people. I mean, it’s kind of like doing a lot of things at the same time. And then we started filming. We shot in New York, LA, Nashville, San Francisco.

Once we did all the interviews, I had my editor and assistant editor in Los Angeles. We were going through all the archives and getting it transcribed and trying to figure out how we wanted to tell the film. We knew that we wanted to tell it with Robin’s voice as much as possible, kind of have him telling his own story. So we were really looking for audio. It turned out that Larry Grobel, who had interviewed Robin twice for Playboy, had kept all of his audio tapes. It was that kind of stuff … always looking for something that people haven’t seen or heard before.

I had an editor named Greg Finton, who I worked with on the Duke lacrosse film for ESPN. And then we ended up bringing in a second editor, Poppy Das, later in the process, because what you really need with this kind of film ― where you have 100 to 150 hours of archives ― is time. You need time to look at things. You need time to pick out audio lines that you’re going to use. You need time to try things out.

Lastly, you did a film on Richard Pryor in 2013 and Robin was in it. Did you get to interview him?

I didn’t. I was sick that day. I was so excited to do the interview and I was deathly sick. I mean, so much so that I couldn’t fly and my producer did it. I felt like that was a missed opportunity and I wonder how this film would have been different had I met him. I don’t know.