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.'"