Dame Julie Andrews has a secret role in 'Aquaman'.

The 83-year-old actress will voice ''the mythic Karathen, an undersea creature that holds the key to Arthur Curry's (Jason Momoa) quest to unite the Atlantean and surface worlds'', in the DC comics adaptation, according to Entertainment Weekly.

Aquaman producer Peter Safran said: ''We wanted the Karathen to have the voice of a classic British actress, albeit somewhat digitally altered. And when we found out Julie was interested and available and excited to do it, casting her was a no-brainer.''

'Aquaman' will go head-to-head with the 'Mary Poppins' remake, starring Emily Blunt. Original star Julie declined to appear in the reboot as she reportedly did not want to distract from Emily's performance.

Safran added: ''Who knew that 'Aquaman' would have the real Mary Poppins this Christmas?''

Meanwhile, director James Wan recently revealed 'Aquaman' will have an optimistic tone, like the critically-acclaimed 'Wonder Woman' film that was released in 2017.

He said: ''I feel like the good thing about having something that isn't really established is I get the opportunity to set the world, set the tone for who this guy is and the world that he lives in.

''That's what we love about superheroes, right? We love that they represent the best part of who we want to be, right, what we strive for and what we aspire to be. And I think what I like most about this character and actually what Jason Momoa brings to it is the idea that this isn't a ... that here is a guy who's kind of trapped between two worlds. He's not ... he doesn't feel like he belongs in the surface world but he doesn't feel like he belongs in the world of Atlantis as well, the underwater world.

''And so I think that's a really interesting ... it gives it more than just a very clean-cut superhero, right? And he's not sort of just out there to just defeat bad guys strictly for that.''