The pair became friends while they were making the 2018 film and sparked speculation about their relationship when they gave an intense performance of 'Shallow', the hit song from their movie, at the Oscars in 2019.
But Cooper, 46, has now explained that he and Gaga were both acting as if they were still in the movie, to help him overcome his nerves about singing live in front of an audience.
“Just from a personal standpoint, it reduces the anxiety level,” he told The Hollywood Reporter.
“They kind of fall in love in that scene in the film. It’s that explosive moment that happens to happen to them on a stage in front of thousands of people. … It would have been so weird if we were both on stools facing the audience.”
The actor also directed the film and revealed that he was struck by the “charismatic and beautiful” singer’s talent.
Cooper said when he met the 'Bad Romance' singer for the first time he realised that if he could “harness” what she had it would only be him that could muck it up.
“But then, when we started working together, I realised, ‘Oh, oh, the sky’s the limit in terms of what she’s able to do and her commitment level'," he said.
The film went on to be a big hit with both critics and audiences and 'Shallow' won best original song at the Oscars.
Watch: Lady Gaga was hospitalised while making House Of Gucci