Things were only elevated during their 2019 Oscars performance where the pair faced each other over a piano as they belted out a rendition of 'Shallow'.
Viewers began wondering whether Gaga and Cooper - who was dating model Irina Shayk at the time - were an actual item.
Now speaking about his Oscars appearance with his co-star for the first time, Cooper told The Hollywood Reporter that they were in fact acting when they performed 'Shallow', which went on to win Gaga the songwriting gong.
According to the publication, he wanted the moment to unfold like a scene from the movie in a bid to help keep his own fears about singing live at bay.
He explained: "Just from a personal standpoint, it reduces the anxiety level."
He went on to say: "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."
He went on to gush over his co-star, adding: "She's just so terribly charismatic and beautiful.
"When I met her, I thought, 'If I can just harness that... then it's just for me to mess up.'
"But then, when we started working together, I realized, 'Oh, oh, the sky's the limit in terms of what she's able to do and her commitment level.'"
Despite this being the first time 46-year-old Cooper has spoken out, Lady Gaga has previously addressed the speculation.
Back in 2019 she spoke to Elle and said: "For me, as a performer and as an actress, of course we wanted people to believe that we were in love.
"And we wanted people to feel that love at the Oscars.
"We wanted it to go right through the lens of that camera and to every television that it was being watched on. And we worked hard on it, we worked for days.
"We mapped the whole thing out-it was orchestrated as a performance."Featured Image Credit: REUTERS/Alamy Stock Photo