When it comes to boxing movies, Sylvester Stallone and Robert De Niro are seasoned veterans. Their new movie, "Grudge Match," has the two facing off in the ring, a first.

The two actors talked with "Nightline" co-anchor Cynthia McFadden about how they prepared for the new film, which, they confide, almost didn't get made.

"I thought it should have been a fight to the death," Stallone said. "But they said, 'Agh, kind of bring the audience down. It's a Christmas film.'"

"Nightline's" Cynthia McFadden hangs with Sylvester Stallone and Robert De Niro for an interview about their upcoming film, "Grudge Match." Credit: Lou Rocco/ABC

Stallone and De Niro have had a friendly competition for years. Stallone, of course, made a name for himself after famously portraying the lead role in "Rocky," which beat out De Niro's "Taxi Driver" for the best picture Oscar in 1976.

A few years later, De Niro took on the role of boxing great Jake La Motta in the 1980 film "Raging Bull."

"Jake was a Rocky in a sense," Stallone said. "When I modeled the character [Rocky] I used a lot of Jake La Motta, his style. … He knows he's going to take five to get one. Indomitable, incredibly arrogant, even in loss, but in a good way."

Sylvester Stallone and Robert De Niro sat down with "Nightline" to talk about their upcoming film, "Grudge Match." Credit: Lou Rocco/ABC

But getting back into the boxing ring was a lot different this time around, especially for Stallone, who said he first had to lighten up a bit about his concerns to preserve the "Rocky" image.

"I didn't know if I was being a little too protective or precious about … if you want to call it a legacy or something that we built over the years," he said.

He told McFadden it wasn't until De Niro himself called that he finally agreed to make "Grudge Match." Between the two of them, Stallone said he was the one who had to work harder to get back into shape for the movie.

"I just didn't feel like working out for a while," he said. "So I just stayed out of the gym a year. So he [De Niro] works out every day."

"Grudge Match" is not the first movie the two stars have worked on together - they teamed up in the 1997 movie "Cop Land" - but even De Niro said he didn't know how this new movie would end.

"They shot it a couple of ways so that nobody knows," he said.

"Nightline's" Cynthia McFadden gets some boxing practice in with Sylvester Stallone and Robert De Niro. Credit: Lou Rocco/ABC