Once Upon a Time
Once Upon a TimeFacebook/Screenshot

Filming of season 4 of "Once Upon a Time" has commenced, and the show's co-creator Adam Horowitz has been busy on Twitter posting snaps from the sets, where the "Frozen" theme is currently being shot.

The first episode of the season will be titled "A Tale of Two Sisters," and undoubtedly it will focus on Anna and Elsa of Arendelle.

Speculations are rife that Elsa will be cast as a negative character in "Once Upon a Time," and co-creator Eddy Kitsis confirmed that that the villain angle will be the heart of the show.

"We are very well aware of how beloved [Frozen] is, so we're very careful that we want to tell a story that is uniquely ours but also honors those characters that everyone fell in love with this past year," Horowitz told TVLine.

Kitsis added: "The thing we love about Elsa is she is perceived to be a villain but is clearly misunderstood, and that goes to the heart of our show."

Recently, Horowitz gave "Once Upon a Time" fans a peek at what Elsa will look like when he tweeted a photo of him watching the monitor on set. The photo shows someone wearing a blue gown, and fans were quick to jump to the conclusion that it was Elsa.

Check out some of Horowitz's tweets below.

"Once Upon a Time" has been really secretive when it comes to the season four plot and filming details, and the cast members, too, seem to be reluctant in giving out too much information on what the season has in store for some of the characters.

Josh Dallas, who plays Prince Charming, recently hinted that viewers will be in for a surprise when the season returns this fall. While Charming and Snow will be seen embracing their roles as new parents, the relationship of their firstborn, Emma, with Regina will be under strain after the former unknowingly brings Robin Hood's love Marian to the present-day world.

"Well, just finished reading the Premier EP to Season 4 of @OnceABC! You won't believe what happens! See ya this fall!" Dallas tweeted.