The veteran screen star wasn't sure the movie was right for him until he heard his actor pal was involved as a producer.
"I met Todd Phillips ("Joker" director) and I liked him," the 76-year-old recalled to People. "I thought, 'This will be an interesting project.' Bradley was one of the producers and he told me to do it."
Robert has worked with Bradley in four films - "Limitless", critically acclaimed "Silver Linings Playbook", "Joy" and "American Hustle" - and he is deeply impressed with the way his younger colleague, who won countless accolades for his 2018 directorial effort "A Star Is Born" starring Lady GaGa, works on set.
"He's really terrific," "The Irishman" star said of Cooper. "What he did with 'A Star Is Born', the way he worked on it quietly by himself, put it together, found the right actors, the right people... When you know what you want to do, that's a great thing, and you know it'll be special."
Cooper has ushered in another award-winning project for De Niro with 2019 release "Joker", which has already earned two Golden Globes for Best Original Score by Hildur Guonadottir and Best Actor for leading man Joaquin Phoenix.
Robert has his own very special prizegiving to look forward to this week though, as he is the recipient of the Screen Actors Guild's Life Achievement Award, which will be presented to him at the SAG Awards by his two-time co-star Leonardo DiCaprio.
"When people in your own profession recognise you in this way, it's an honour," he smiled.
The 26th Screen Actors Guild Awards take place in Los Angeles on Sunday, January 19 at the Shrine Auditorium.