The former fighters are set to get back in the ring to settle the score, and these two have been in the ring before, with Stallone as Rocky Balboa in "Rocky," and De Niro as Jake LaMotta in "Raging Bull." Both trained to box again.
"Yeah, I mean, I worked with somebody that Sylvester's been working with about 10 years, Bob Sally, terrific trainer, really great. And he choreographed it and he'd bring the choreography to me," said De Niro. "It was fun. I enjoyed it."
Stallone has his own technique for developing boxing scenes.
"I actually take highlights from great films and say, 'OK, this is obviously work. Is he gonna get knocked out?' And borrow that and build on it. So everything that is in the film is an actual movement from real fights that took place from the 50s on."
The guys, who know their way around R-rated material, are happy for a little PG-13 holiday cheer.
"And this one is a revelation. It's really good. I mean really, really good. I'm not just saying that. It's just a great feeling movie," said Stallone.
The guys also spar outside the ring. In the film, one of their fights goes viral. Neither character has a clue about what means, and in real life, neither star is all that savvy in the world of social media.
"No, I mean, I know a little bit about the terminology because my daughters, they live on it, like Vines," said Stallone. "Next movie we do has to be a Vine. It's six seconds long. I love that idea."
"Grudge Match" opens Christmas Day.