20th Century Fox

Julie Andrews has a role in a big Christmas release, but it's not "Mary Poppins Returns."

The actress, who won a Best Actress Oscar for 1964's "Mary Poppins," has a secret role in "Aquaman," according to EW.

She'll voice Karathen, an undersea creature that holds the key to Arthur Curry’s (Jason Momoa) quest to unite the Atlantean and surface worlds.

"We wanted the Karathen to have the voice of a classic British actress, albeit somewhat digitally altered," explained 'Aquaman' producer Peter Safran. "And when we found out Julie was interested and available and excited to do it, casting her was a no-brainer... Who knew that Aquaman would have the real Mary Poppins this Christmas?"

Andrews declined to cameo in the new Mary Poppins film, saying she didn't want to detract from Emily Blunt's performance.

The "Sound of Music" star's last onscreen film role was in 2010's "The Tooth Fairy." She's also lent her voice to "Despicable Me 3" and "Shrek Forever After" and she  appears in the Netflix series "Julie’s Greenroom."

"Aquaman" and "Mary Poppins Returns" both open on December 21.

[Via EW]