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

Bradley Cooper and Lady Gaga think they put their careers at risk with 'A Star is Born' because it could have caused them to "turn a bad corner" if it hadn't gone well.

The Oscar-winning actor has moved behind the camera to direct their upcoming musical, while the 'Poker Face' hitmaker is making her big screen debut, and though the film has received rave reviews so far, both stars were aware it could have been their downfall.

Bradley said: "There are certain projects where you can go, 'It didn't work out, but it's OK'.

"And there are others where it's like, 'Actually, that wasn't OK, and I took about five steps back.' This was one of those. If it didn't work, we both knew it was going to affect our careers."

Gaga agreed: "Things could have turned a bad corner."

But the 'Silver Linings Playbook' actor admitted a lot of his career decisions - such as starring in the stage revival of 'The Elephant Man' in 2014 - haven't made "sense".

He told the Sunday Telegraph magazine: "If I had been thinking about this like a corporation, there would have been a big meeting after I finished 'American Sniper'.

"And we'd all be like, 'OK, this is now our shot at the prime time, and we don't know how long it's going to last.' 

"And we would not have then decided to do a play and then make 'A Star Is Born' for the fourth time. It makes no sense."

But Bradley realised the movie was the ideal project for his directorial debut when he was initially preparing to star in the movie, which initially had Clint Eastwood at the helm.

He said: "[I remember] suddenly having this specific point of view deep inside me about what this love story was. And I know it sounds sophomoric, but I would only know how to tell a story if I had a point of view. 

"Some people said maybe I should direct a commercial, or a pilot for a TV show, but that would have scared the daylights out of me."