Just in time for Father’s Day, Robert De Niro is losing his tough exterior for a revealing new HBO documentary about his late father, Robert De Niro Sr. — a struggling artist who was also openly gay.

In an emotional Out magazine interview, De Niro opened up about his father’s legacy and the responsibility he feels to tell the elder De Niro’s story in the most honest way possible. “I felt I had to. I felt obligated. It was my responsibility to make a documentary about him,” the Academy Award winner says. “It was not intended to be on HBO. It was just something I wanted to do.” Originally, De Niro intended simply to make the film for his own kids, using archival footage from the ’70s with the help of Martin Scorsese’s editor Thelma Schoonmaker.

Though De Niro’s parents divorced when he was only three years old, his father still lived within walking distance of him in the Greenwich Village and Little Italy areas of Manhattan — though the two were not close. “We were not the type of father and son who played baseball together, as you can surmise. But we had a connection,” explains De Niro.

In the film, De Niro gets emotional while reading excerpts from his father’s personal diaries. The passages revealed De Niro Sr.’s struggle with his sexuality. When asked whether he thinks his father felt conflicted about being gay, De Niro tells Out, “Yeah, he probably was, being from that generation, especially from a small town upstate. I was not aware, much, of it. I wish we had spoken about it much more. My mother didn’t want to talk about things in general, and you’re not interested when you’re a certain age. Again, 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.”

Though De Niro himself has never played a gay character, the actor insists this has nothing to do with his personal past. “No, they weren’t offered to me. If they had been offered to me by a good director, that’s something I would have considered.”

You can read the full interview here. Remembering the Artist: Robert De Niro Sr. premieres on HBO June 9.