Another day, another exciting piece of Once Upon a Time casting news.

First, we learned that Georgina Haig has been cast as Queen Elsa on the ABC hit.

Then, the network confirmed that Elizabeth Mitchell will play an unknown role this fall.

And now TV Guide reports that Giancarlo Esposito will reprise his role as Sidney Glass (aka the Magic Mirror) for an unknown number of upcoming episodes.

The last time fans saw any reference to Sidney, it was on the nameplate of a locked door in the deep recesses of Storybrooke hospital.

The Once Upon a Time Season 4 premiere will be titled "A Tale of Two Sisters” and it will introduce Frozen characters Anna (Elizabeth Lail), Elsa (Haig) and Kristoff (Scott Michael Foster).

Esposito, of course, also appeared for two seasons opposite Mitchell on NBC's Revolution.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.