Cooper is directing the musical romantic drama remake of the 1937 film "A Star Is Born", with Gaga in the lead role.
At the Tribeca Film Festival panel with his "Limitless" co-star Robert De Niro, Cooper opened up about the pact that he and Gaga made before the cameras started rolling, reports variety.com.
"She said right from the beginning that this was going to be a bargain," Cooper said.
"'I'm gonna rely on you to get a performance that's honest out of me', because she'd never done a film before, and 'I'm gonna make sure you turn into a musician, because we're going to sing everything live'.
"And I thought, wait, what? She said, 'No, the only way this is going to work -- I can't stand when I watch movies when they have music and you can tell when it's pre-recorded and people are lip-syncing'. And she's right.
"So that was terrifying, but I really relied on her."