The relationship between the two mismatched cousins played by Jesse Eisenberg and Kieran Culkin in A Real Pain is balanced by the simple yet sophisticated score that consists almost entirely of piano pieces by Polish composer Frdric Chopin a purposeful decision by Eisenberg, who also wrote and directed the film.

As I was writing A Real Pain, it was focused on this funny and fraught relationship between these two guys that were trying to connect to their past relationship, and in many ways, they became children around each other, Eisenberg tells THR of his Holocaust-history dramedy. And I wanted the tone of the movie, very specifically, to not be juvenile. I wanted the tone of the movie to be a commentary on these characters falling into old patterns against the backdrop of historical trauma. I wanted the movie to have a sophisticated, traditional, mature look and feel to it. Eisenberg started plotting the inclusion of Chopin tracks, mostly nocturnes and tudes, while writing the script. I was just listening to all these pieces, and a lot of times they were inspiring me for what the scene should be, he says. Im sitting at the library writing, and its just playing on an entire loop for hours in my head, and then Im playing it when Im putting my kid to sleep, and Im playing it as I get ready in the morning, because Im trying to put myself in the spirit of that kind of emotion.

Sometimes he even let the speed of the track inform the way a scene was shot. For example, when the cousins are in the small town their family comes from, Eisenberg played the track he chose for that scene for the dolly grip so that the grip would push the dolly to the beat of a Chopin nocturne.

Eisenbergs foray into Chopins music started with his 2008 trip to Poland to see where his family originates. He paid a visit to Chopins house in Zelazowa Wola. After Eisenberg returned, he wrote a stage play called The Revisionist in which he needed interstitial music, so he chose Chopin. Ten years later, he went back to the composer for A Real Pain. (The films music, performed by Tzvi Erez, is not eligible for the Oscar because its not an original score.)

While we were editing the movie, some producers were getting a little nervous about the accessibility of this movie for young people, and I was encouraged to explore more pop music on the soundtrack, he reveals. My argument was always, This was in the script. This is what we all had agreed upon, so why change it now? Lets take this risk. Its not overwhelming. The music is not jarring in any way. Lets commit to what this is.

But one piece of music in the score is not Chopin: a cover of Slim Smiths reggae song My Conversation, which Benji (Culkin) listens to in the shower. The track also plays over the end credits.

My editor, Rob Nassau, whos great, he just felt like the movie could use a little bit of lightness there, he says, referring to the end credits. In retrospect, I probably would have kept it with Chopin.

This story first appeared in a January stand-alone issue of The Hollywood Reporter magazine. To receive the magazine, click here to subscribe.