Culkin, 42, stars alongside Eisenberg, 41, who grew up in East Brunswick and wrote and directed the film.
"Thank you for this movie," he told Eisenberg in his Oscars acceptance speech Sunday. "You're a genius. I would never say that to your face, I'm never saying it again, so soak it up."
READ MORE: N.J.'s Jesse Eisenberg talks 'A Real Pain,' inherited trauma and his family promise
Culkin also made a callback to his winning speech at the Emmys, when he said that his wife, Jazz Charton, had promised him a third child if he won. At the time, he told her from the stage that he wanted that third child. Charton apparently got some unwanted attention over the remark.
"Turns out she said that because she didn't think I was gonna win," he said.
But Culkin later told her that he actually wanted four kids. With that, Charton made another promise.
"She said 'I will give you four when you win an Oscar,'" Culkin said with a smile.
His character in "A Real Pain," Benji Kaplan, is an unpredictable, outwardly emotional person, while Eisenberg's David Kaplan is his buttoned-up, more reserved relative.
Culkin is charismatic and a magnet for attention as Benji, a chaotic member of their Holocaust history tour group in Poland. He also wears his pain on his sleeve and is known to create a scene, leaving his cousin David to clean up after him.
Eisenberg spoke to NJ Advance Media about "A Real Pain" before the November release of the Searchlight Pictures film.
He said Culkin's performance astounded him.
"When I first started watching him, I guess I was just immediately struck by this guy, this actor, who I had literally never spoken to about the role -- we never talked about the character -- and he started performing it, and I just remember thinking 'he's doing exactly what was in my mind for the last three years,'" Eisenberg said.
He took inspiration for his real family history in Poland to write the story. He and Culkin filmed at his real great-aunt's old home in Krasnystaw, Poland.
In 2003, he was nominated for a Golden Globe for his performance in the movie "Igby Goes Down," winning a Critics Choice Award for best young actor.