Robert De Niro, Sylvester Stallone on Making 'Grudge Match'

They just may be the two greatest fighters in movie history, and now they're settling the score.
3:00 | 12/18/13

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Transcript for Robert De Niro, Sylvester Stallone on Making 'Grudge Match'
They might be two of the most famous on screen boxers, sylvester stallone as underdog ricky balboa and robert de niro as raging bull boxer, the two go at it on the screen for "grudge match" they put on the kid gloves with me for their "nightline" interview. Your punch. There is not much that can match the thrill learning how to throw a punch from the most iconic movie boxer of all time. But it gets even better, buzz to my right the other most famous big screen fighter of our generation. Push with your shoulder. Um. Whoa. Reporter: In one corner, sim vester sta len as rocky. In the other corner, robert de niro as jake lamata as raging bull. Now all the years later they're in the ring together in the movie "grudge match." The concept two old rivals decide they need to settle a score please I am becki ibegging you. Begging you. Reporter: Throwing punches at each other and taking a few jabs at the roles that made them famous. Whutz i whose idea was it? Wasn't my idea. It is fooling with two boxing icons of our generation, the generation before, and probably the next generation. It's raging bull and rocky. And head-on collision. He is going to be cartoonish, raging bull winkle, rocky the flying squirrel. I don't know. Reporter: Stallone is protective of his iconic roll, won three oscars, spawned five sequels and often named as one of the best films of all time. Palgsed f edpassed for a year. Called you another year. Wasn't all till he called. Couldn't get him off the phone. I just thought an interesting thing for us to do. I said don't worry it will be okay. How do you know? I know. Reporter: A leap of faith kidding they had a real life rivalry dating back 40 years when rocky faced off with taxi driver for best picture. Rocky and taxi driver came out the same year. They were against each other. Competing. In the ols carscars. Here we are. In '76. All most 40 years later in the ring. And the winner is "rocky." ♪ to all the rockies in the world I love you. Reporter: Since newt fithe new film is called grudge match. We wondered. Do you hold grudges? I don't really hold grudges. If I hold them. I get over them. I've don't like to keep that energy in myself towards other people. Reporter: How about you? Grudges? Are you serious? Yeah. For real? So we like each other now. Jimm Reporter: You two could be the new odd couple. Could be something in this. Sheriff, sorry to have awoken you from your slumber. It is over. Roipt two h Reporter: The two have teamed up before in the 1997 movie copland. Getting into the boxing ring was a lot different. Especially for sta len who says he first had to lighten up a little bit. Don't know if I've was being too protective or pressured, about the legacy, or something that we built. Reporter: Finally he was ready to go there. Even willing to take a swipe at one of rocky's memorable scenes. Hey, hey, what are you doing? Just here to buy a little dinner. You didn't have to punch everything. Who was the most out of shape? Me. Reporter: Really? Yeah. I just didn't really like working out for a while. I just stayed of the gym, a year. He works out of every day. Reporter: You're tough. No, no, no. Reporter: The punching looks like you are both getting hit. Body shots here and there. Reporter: Nobody got hurt? No. Thought it should have been fight to the death. They said, no. Bring the audience down. Ach christmas film. That is stallone. Big, bold, grabbing the world by the throat. De niro, reserves, quiet, intense. I wonders where these qualities came from. Just my upbringing. My father was, pound for pound, just a tough person. I picked up some very bad habits. Even to this day I am still unlearning things. There will be moments where i will get angry and still. I hear him. I mean, really I have the same speech pattern. And my wife will bring it up. Your father is back. I go, ah. Reporter: What did you learn from your father? To me he had a lot of principles. He was very clear about what he felt. He was an artist. Yeah. He just always did what he thought he should do. What he believed in. He didn't, when things changed when the art scene changed. What was coming out he kept doing this thing. I came early. Want to see my old friend. Duck me for 30 years. I missed him. Reporter: I love the beginning you are going to make the video version of the fight. Go ahead. Reporter: You come out. I think we can agree to call it a ridiculous costume. Leap of faith, okay. What are you doing? No, no. Reporter: We sat down with them on a snowy new york morning, surrounded by photographs of fighters from the past. On behalf of movie fans and fight fans, if rocky got into the ring with jake, what would be the outcome? Pain. Reporter: I am serious. How would that come out? Called 911. First of all, jake was rocky. When I modeled the character. I used a lot of jake, right there in your face. Knows it will take five to get him in. As for who wins in the new movie you will have to go to the theater to find out. They shot it a couple of ways, so nobody would tell any body. I don't know the answer. In the film, a great performer knows when it is time to exit the stage. You guys are far away from that now. Is it something you think about. I would like to add to that. A great performer knows when it is time to exit the stage. Don't do it. What's the rush? What's the alternative. What any the alternative? A better one. Let's hope neither one of the greats find an alternative anytime soon.

This transcript has been automatically generated and may not be 100% accurate.

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