Selena Gomez still can't understand what her character is talking about in new movie The Big Short, because she can't get her head around collateralised debt obligations.

The singer/actress was an odd choice to play a financial expert in director Adam Mackay's new movie about the credit and housing bubble collapse of the mid-2000s, and she admits she had no idea what she was talking about.

"The producers called me, and I even said to them, 'I have no idea what I just read'..." she tells Rolling Stone. "I just had to learn so much so fast. It's a great movie."

Selena admits she's glad her scenes were with economist Richard Thaler and not with the film's stars, Brad Pitt, Steve Carell, Ryan Gosling or Christian Bale.

"I would have been bright red and sweating the entire time," she giggles.