Gianni Russo had an excellent reason to avoid the 45th anniversary screening of “The Godfather” at Radio City Music Hall, where Robert De Niro, Al Pacino, and the rest of the cast were in attendance.

Russo, who played the ill-fated, abusive husband of the only Corleone daughter, says his relationship with James Caan had frayed before filming began, but their tension reached a head while filming the violent altercation on the streets of Harlem in New York City (sparked by Rizzi’s mistreatment of Corleone’s sister).

Caan improvised once the cameras were rolling, shattering two of Carlo’s ribs, despite the fact that the performers had rehearsed the sequence and it was perfectly scripted.

“Sonny and I had a problem all through the movie, on and off the set,” Russo tells People and Entertainment Weekly Network, adding that his familiarity with the area (and its people) “pissed [Caan] off” before the cameras rolled. “We choreographed that scene for about a day and a half… Jimmy got a little aggressive, I would say, and he improvised a few things like that little billy club he threw at me when I came off the stoop. He hit me right in the head with that, and then he throws me over the railing and he’s biting my hands… when I crawl out, he literally lifted me up with his kick… none of that was supposed to happen.”

Russo grew up in Manhattan’s Little Italy in the 1940s and was familiar with real-life mafia leaders, many of whom were outraged when a Godfather picture was announced. Russo served as a go-between between mafia leaders of the time and the film’s producers, ultimately assisting the film’s production.

Nonetheless, he claims he planned to keep the drama on the big screen.

“With this being my first film, I didn’t want to complain,” Russo explains. “Jimmy and I are not friends at all, believe me. The guy’s nuts.”

It was Russo’s first film part, and he had the good fortune of being trained on set by none other than Marlon Brando. “I was never in front of a camera, didn’t know protocol on the set,” Russo explained. “Brando became my friend, he mentored me through all those scenes, taught me all those great nuances.”

Among all the beatings Caan delivered to Russo, there is one that stands out as clearly orchestrated.

“That one miss when he throws that punch at me and I react… everybody knows [the punch is actually] about six inches away from me,” he recalls. “I asked Francis Ford Coppola at the 25th anniversary, ‘Why didn’t you fix that?’ Because we had cameras on the roof, cameras all over. We had every angle they needed to fix it, but they couldn’t touch it. Once you win an Oscar, you can’t touch that negative.”

Caan, on the other hand, denied that he and co-star Gianni Russo had a falling out on site, saying that Russo was wrong.

“He had a fight with someone else. Not me,” Caan said. “I did the fight scene with stuntman Paul Baxley. He came in, and we made up the whole fight. And everything you saw in there is something that Paul and I created the day before.”

Russo, who appears in a flashback in “The Godfather Part II” and 35 other films, will celebrate his 45th birthday on May 10 at Beautique, where he performs every Wednesday night. For guests who are sipping Don Corleone vodka, he will sign stills from the film. “I’ve made more money than anyone from ‘The Godfather,’” he said. “Who thought it would last this long?”