The Taxi Driver star plays a con man who encounters his drug addict son Nick Flynn, portrayed by Dano, working in a homeless shelter in Boston, Massachusetts.
The opening scene sees De Niro answer the door to the Little Miss Sunshine actor after climbing out of a bathtub - and his lack of clothing left Dano red-faced.
The 27 year old tells WENN, "I think he was (naked) but he was covered in suds (soap bubbles)! I tried to avert my eyes. Bob went for it. He was really committed and prepared.
"I think he's amazing in the film. We don't totally get along in the film so Bob was very warm and welcoming and kind to me off set, but when we were at work we got to business. There wasn't a lot of hanging around the craft service table. I wasn't calling him Bobby D by the end of the movie but he's a sweet guy!"
Being Flynn is based on the 2004 memoirs of Nick Flynn, titled Another Bulls**t Night in Suck City.