The cast shot the upcoming romantic drama in Cooper's native Philadelphia, Pennsylvania, and in order to best prepare for the part, De Niro asked the actor's relative Ernie for vocal tips.
Cooper tells U.S. TV host David Letterman, "Often you see movies in Philadelphia where everybody sounds like they're from New York... But he really wanted to get that Philly accent down... (so) Robert De Niro said, 'Bradley can I have your uncle read my lines?' I thought, 'Oh my God.'
"So we're in a hotel room, my uncle Ernie is sitting there on the couch, then Robert De Niro is next to him, and (director) David O. Russell is next to him... Then David O. Russell started to direct him... It was so crazy. They went through the whole script!... At one point I literally could not stop laughing, because this is my uncle, like this tough guy, he's got hands like catcher's mitts, just being sort of attacked by both sides by these guys and just sweating...
"But the Philadelphia mentality, they are pretty confident, and we walked out and he (my uncle) said, 'We'll see if he does it as good.'"
The Irish actor admits he only learned to "experience himself" more recently after sobering up and becoming more responsible following years of body abuse.
Nowadays, fans are more likely to see him out jogging or at the gym than partying with his peers - and he insists his hard work has paid off.
He tells WENN, "With exercise there's nothing going in, but sweat coming out. There's so many ways as human beings to experience ourselves. That's just a way that I've grown in the last four years to really enjoy experiencing myself, feeling my body in space and actually being aware.
"For years if you asked me how many fingers do you got (sic) on your hands, chances are that I didn't have to look down and start counting but I wasn't that aware. It's kind of really incredible and sometimes it's really painful and sometimes it's really joyous and sometimes it feels unfortunate and sometimes it feels blessed, but working out is something I've enjoyed doing the past couple of years and I love it.
"Sometimes I need to get out of the house (and work out), but if anything it's just an escape to another aspect of myself. For (new film) Total Recall I worked with the mitts (gloves) and I got really fast jogging. I was good in school in the 100 metre (dash) but never a long distance runner. But I got three miles down to a good time."
The actor/director hired a stellar ensemble of action stars including Bruce Willis, Mickey Rourke and Arnold Schwarzenegger for the upcoming film, which follows the story of an elite group of mercenaries sent to overthrow a ruthless South American dictator.
Stallone jokes that the ageing Hollywood hard men are well past their prime, but he's "proud" to have gathered such an "ugly" group of actors.
He tells Fox News, "I thought we had the ugliest group of guys on the planet in one movie. That is what I am so proud of. We all look like horseshoes or catchers' mitts.
"People go, 'Wow, that is a sexy group of guys'. And I say, 'Excuse me, you're talking about the Twilight team, not us.'"