Actor Bradley Cooper is filmed as he performs on the Pyramid Stage at Worthy Farm in Somerset during the Glastonbury Festival. Picture: Reuters

Chart-topping singer Lady Gaga convinced Bradley Cooper to sing live in 'A Star Is Born'.

The 32-year-old star appears alongside Bradley, 43, in the upcoming romantic-drama film, and Bradley - who directed the project - has revealed she implored him to sing live in the movie in exchange for help with her acting.

READ: Lady Gaga thanks Little Monsters on Just Dance's 10th anniversary

He said: "[Gaga] said right from the beginning that there would be a barter. 

"[She 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 that you're going to turn into a musician. Because we're going to sing everything live.' And I went, 'Wait, what?'"

Although Bradley spent a long time preparing to sing, he still found it to be a "terrifying" experience.

He shared: "[Singing live] was terrifying, and I really relied on her. I spent about a year and a half taking vocal lessons and preparing."

The Hollywood star actually spent three years preparing for the 42-day shoot.

The movie is a remake of the 1937 film of the same name, and Bradley revealed he was determined to helm the project, regardless of whether it was a success or not.

Bradley told Vulture: "The broken love story in 'A Star Is Born' kept haunting me. 

"Shots kept coming into my head. I would dream about it. I realised I had to [direct it], whether it fails or not. I knew I had to try and I wound up absolutely loving it.

"You can't hide when you sing. The best way to express love is through singing and music. I knew if I could marry that in a way, it would be special."