Actor Bradley Cooper poses with Lady Gaga at the world premiere of A Star is Born at the Toronto International Film Festival (TIFF) in Toronto. Picture: Reuters

According to Lady Gaga, she "knew instantly" that Bradley Cooper could play a musician in 'A Star Is Born'.

The 32-year-old singer appears alongside Bradley in the new musical romantic drama, and Gaga has revealed she was quickly convinced of the Hollywood actor's singing credentials.

Recalling how he pitched the idea of making the movie to Gaga, Bradley - who also directed the film - explained: "She was kind of laughing at me that I would be suggesting this, but I said, 'The truth is, it's only going to work if we can sing together.' And she said, 'Well, what song?' And I said, 'Midnight Special'."

Gaga explained: "I printed out the sheet music, and he had the lyrics on his phone, and I sat down at the piano and started to play, and then Bradley started to sing and I stopped: 'Oh, my God, Bradley, you have a tremendous voice.'"

Although Bradley wasn't previously known for his singing talents, he didn't take long to convince Gaga that he was perfectly suited to the role of Jackson Maine in the new film.

And the chart-topping star feels they've managed to create "something truly special".

She told Vogue magazine: "He sings from his gut, from the nectar! I knew instantly: This guy could play a rock star. And I don't think there are a lot of people in Hollywood who can. That was the moment I knew this film could be something truly special."