De Niro and Keitel previously appeared together in the film classics "Taxi Driver" and "Mean Streets."
In their latest cinematic outing, Keitel plays a contractor trying to take advantage of new homeowner Greg Focker, played by Ben Stiller. De Niro plays Jack Byrnes, Greg's intimidating, frequently meddling father-in-law, who intervenes with the builder on Greg's behalf.
"It seemed like a good thing for Harvey to be in it and everyone was OK with it, and that's how it went," De Niro told UPI at a recent New York press conference.
"It was fun. When you have different people with different styles of working sometimes you've got to get around that stuff, and Harvey's one of my oldest, dearest friends, so we had to get all that working. It was good though, terrific."
Asked what has kept him coming back to the "Fockers" franchise through three movies, De Niro replied, "We have fun."
"I enjoyed it a lot," he said. "When you're doing comedy, I have less restrictions or constrictions. You can either just cut the thing out or the director, Paul (Weitz) in this case, will say try something else or that's not working. There's more room for just having fun in a certain way."
One of the most memorable scenes in "Little Fockers" features Greg and Jack fist-fighting in a ball pit at a children's birthday party.
"Obviously, it's a movie fight," Stiller noted. "The ball pit stuff was all real. It's a very strange environment to be in, a ball pit, and to be smothered. And it's not very hygienic either. Ball pits are just little petri dishes, or I guess a giant petri dish, but that stuff was all really fun.
"The bouncy house was challenging," Stiller added. "I pulled a muscle. I'm not going to say which muscle, but I pulled a muscle. It's hard to get your equilibrium. Those are environments that are made for children, not older gentlemen like ourselves."
"Little Fockers" is in theaters now.