McKay, 47, revealed that while writing the script, he would explain the banking world to his kids to make sure it was understandable enough for viewers.
"I practiced on Pearl and Lily Rose, and I would tell them how banking worked, and at one point, they were like, all right," the Oscar nominee told E! News while walking the red carpet.
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He also said Pearl, 10, and Lily Rose, 15, are to thank for Selena Gomez's surprise cameo in the film.
"I'm not going to lie to you, definitely Selena Gomez – I was thinking of my daughters," he said of his decision to cast the pop singer.
In fact, he figured including a few big-name celebrities would help make the often complicated content of the film more relatable.
"I thought I could skewer pop culture by having Selena Gomez, [Anthony] Bourdain, Margot Robbie – and at the same time, explain this stuff."
The Big Short is nominated for the highly-coveted Best Picture award, while McKay is also up for Best Director.
But just because the film was widely successful, doesn't mean the finance world is a big fan of the director.
"I'm not allowed at banks anymore." he joked.
The 88th annual Academy Awards, hosted by Chris Rock, are being presented live on ABC from the Dolby Theater in Hollywood, California.