The drama follows Bradley Cooper's unstable Pat Solitano as he struggles to cope with bipolar disorder while forming a friendship with a newly-widowed dance enthusiast, played by Jennifer Lawrence.
Russell was inspired to shoot the film by his teenage son Matthew, who suffers with the condition in real life, but when he asked his dad if he could play a small role in the movie, the filmmaker insisted he try out like any other actor.
He says, "He's acted in plays at his school, it focuses him, it inspires him. And I said, 'If you're gonna do this, you gotta do good in school, you gotta do good with your behaviour and you have to audition, because there's nothing easy about this business. I don't wanna give you the wrong message.'"
Russell's son Matthew landed the role as awkward Ricky D'Angelo, but the director admits there were challenging moments with his kid on set.
He explains, "(In one scene) Jennifer Lawrence has really shaken up Bradley Cooper, he had a big bipolar explosion and he and (Robert) De Niro were fighting in their pyjamas. And then the doorbell rings, and... it's this nosy guy. And De Niro... opens the door... and he pushes my kid, and my kid explodes into nervous laughter.
"I said, 'Wait a minute. Cut. What are you doing?' I said, 'This is a serious scene, Robert De Niro is pushing you.' He said, 'My God, but dad it's like I'm in Raging Bull, he's like coming at me.' I said, 'That's what you signed up for, so you gotta deal with it. Come on, let's go.' It happens take after take, he's laughing nervously. And finally De Niro... he says to us, 'Listen, it's OK, let him do this, it's real.'"