The "American Sniper" actor makes his directorial debut with the film, in which he also stars as a washed-up alcoholic rocker, and admits he tapped into his past to help him get into character.
"Anytime you're trying to tell the truth you need to go to places and use things that have happened to you, or you've read about or experienced," he told Variety at the Toronto International Film Festival in Canada, "and that's all part of the beauty of turning whatever things you've gone through into a story."
"I find that to be very cathartic. I remember learning that in grad school - our teacher said all the insecurities, all the dark stuff you get to use that and that's really the truth."
Bradley stars alongside Lady GaGa in the film and, like the "Born This Way" hitmaker, the actor sang all his own tunes and played the guitar. He reveals he prepared for his musical parts by taking guitar lessons and was thrilled he was able to live out his rock star dreams on the set of the project.
"Because she's (Gaga) so confident and supportive, I felt pretty comfortable once I got there," he said. "It gave me the guts to go out."
The pair hit the stage at the Glastonbury festival in England and performed in front of 80,000 people for a scene in the film and Cooper also managed to land a guest spot during Willie Nelson's set at the Stagecoach country music festival in California.
"It was nuts, man," Bradley beamed. "As a kid you always dream of being a rock star, so to pretend to be was pretty cool."