Robert De Niro is known to be very discreet when it comes to his private life, but in a rare occasion he recalled past memories of his late father.

The 70 year-old iconic actor is currently involved in a project which depicts the life of his father, who was openly gay, called 'Remembering the Artist: Robert De Niro Sr.'

De Niro Sr., who died in 1993 from prostate cancer (71), was an abstract expressionist painter, whose work was part of the movement in the post WWII art scene.

He was successful in the 1940s and 50s but as time passed, so did the style, and as the pop-art of the 60s rolled in, De Niro Sr. began to struggle.

"I felt I had to. I felt obligated. It was my responsibility to make a documentary about him," he said in an interview with Out magazine about the documentary, which was edited by Thelma Schoonmaker, a longtime collaborator with famed director Martin Scorsese.

Although De Niro wanted to honor his father with this project, the pair did not live together since his parents divorced when he was just three years old.

"We were not the type of father and son who played baseball together, as you can surmise," he said. "But we had a connection."

The 'Raging Bull' actor also plays a part in the doc, he becomes emotional when reading out of his father's journals, which reveals that his parents separated after De Niro Sr. realized he was gay.

"Being from that generation, especially from a small town upstate," De Niro Jr. told Out about his father's upbringing. "I was not aware, much, of it. My mother didn't want to talk about things in general, and you're not interested when you're a certain age."

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But not speaking about it with his father, who passed away over 20 years ago, is still something De Niro regrets to this day.

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"I wish we had spoken about it much more," he said of his father being gay. "For my kids, I want them to stop and take a moment and realize that you sometimes have to do things now instead of later, because later may be 20 years from now --- and that's too late." The documentary will air on HBO on June 9th.

Robert De Niro
Robert De Niro and his wife, Grace Hightower, at the 2014 Tribeca Film Festival