Elf star Zooey Deschanel reveals the story behind the 'Baby, It's Cold Outside' scene

The pivotal duet wasn't in the original script but was tailored to Deschanel's skill set. "I remember [director] Jon Favreau telling me that they were catering it to whoever played the part," Deschanel tells EW. "One actress they were looking at was good at skateboarding. But I had a cabaret act at the time and I was performing a lot. They knew that I was a singer, so they put that in to be my special thing that he could discover I was good at."

For Deschanel, the real discovery was Ferrell's pipes. "He said he wasn't a singer, so I wasn't expecting much," she recalls. "But his dad [Roy Lee Ferrell Jr.] is a great musician. I should've known he'd be secretly good!"

Ferrell cuts through the song's controversial quality and shower setting too. "It's funny because obviously everyone knows it's not appropriate to walk in the girls' bathroom when someone's showering, but he's so believable as this guileless elf," Deschanel says. "It's weird he's in there, but you totally buy that his intentions are pure and innocent."

Deschanel was still relatively new to acting at the time, so she was surprised to learn that they had to pre-record vocals, rather than performing the duet live on set. "They recorded some live vocals as an option, but they almost never use those just because you just can't make it right," she says. "I'm in a shower, there's nowhere to hide a microphone, and then they wouldn't have a clean vocal at all. It would be completely mixed with the sounds of the shower."

Favreau selected the tune as Deschanel recalls, but as a lover of the standards and a fan of the Ella Fitzgerald version, she was excited to tackle it. She went on to record the song with Leon Redbone for the soundtrack. "I was a big Leon Redbone fan [and] had bought a Leon Redbone CD when I was 11 years old [through an NPR fundraising drive]," she says, laughing.

To lay down the soundtrack version, Deschanel went into the studio with Redbone, but he had already recorded his part, leaving her to match her harmony to his voice. "I had to do it without Leon because apparently he's very shy, and he didn't want to do it at the same time as me," she explains. "So I just had to guess how long he was going to hold [the final note]. When he recorded his, he held his out a lot more than me. It sounds good, but I wish I'd known he was going to hold it out longer."

Deschanel later recorded a gender-flipped version on a She & Him holiday album. "I was a little hesitant to do it because I had done it," she admits. "So I said, 'How about we flip the script and I'll sing the male part?' So that's how we ended up doing it." When it comes to the controversy, she views the song as more of a "character study," and a chance for two vocalists of any gender to explore two distinctly different characters.

She also credits Elf with the song's resurgence and says, "It hadn't really been revisited. [A few] big names recorded it in the '50s, now everybody records a version."

Because the best way to spread Christmas cheer is singing loud for all to hear.

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