WATCH: How Robert De Niro became 'The Irishman'

Robert De Niro is no stranger to Martin Scorcese films. The longtime friends have collaborated on nine films, the latest being the Oscar-nominated "The Irishman." But this new meditation on wiseguy life does something no other Scorcese film has done: places De Niro alongside Al Pacino and Joe Pesci in one universe.

"I don't know if we would ever be doing another movie together like this, and so, it felt right," De Niro said of the film. It reportedly took nearly 15 years to get off the ground after a series of delays and hurdles, including coaxing Pesci, who hadn't acted in years, to join in.

De Niro plays Frank Sheeran, the titular "Irishman," who gets wrapped up in a life of mob violence until all he has left is a sense of resignation — but not remorse — by the end of the film. He's seen in a wheelchair in a nursing home, recounting his mafia tales to caretakers who don't know who Jimmy Hoffa is (not just a towering figure in American history, but a main character in the film).

The film's final moments of Frank's passivity echo a line that's uttered earlier in the film: "It's what it is."

De Niro said Sheeran is a "guy who's getting on. He's wasting away. He wants to have this final, if you will, confession about what he did."

De Niro told the PBS NewsHour that he fuses elements of himself into the characters he plays. The actor said it was a "conscious process," that's also encouraged by Scorcese.

"That's what's great about Marty, because he'll say, 'Yeah, go ahead and try it,'" he said.

De Niro said there's a certain "mystery" to acting as a craft. "Every actor works differently at the same time," he said. "I think probably we all have something in common about how we approach a part, and I always feel that whatever works, as long as you don't hurt yourself, or anybody else."

The PBS NewsHour's full interview with Robert De Niro will air later on this week.