Shazam, formerly known as Captain Marvel, is just one of many superheroes whose lives in the world of DC Comics, but the key thing that separates him from the rest of his allies is that In his normal, everyday life, Billy Batson is a kid. Oh sure, there are plenty of younger superheroes in the DC universe, but in Billy's case, he becomes a mighty, adult protector of the innocent after saying the magic word, "Shazam!" It's one of several reasons why the character has endured for so long, and as Zachary Levi, who will play Shazam on the big screen, sees it, the Shazam movie will basically be the DC Extended Universe's version of 1988's Big. As Levi put it:
But I can tell you that I'm out of my mind excited for this project! I love that I get to play a superhero who is really a 14-year-old kid inside. I've always been a fan of the movie Big, and I feel like this is my Big. That I get to do it as a superhero is double trouble awesome.
On the off chance you've never stumbled upon the Tom Hanks classic Big (it's really worth a watch), it follows a young boy named Josh Baskin who is desperate to be "big." After a spooky encounter with an arcade fortune teller machine, his wish is granted, and he awakens the next morning and discovers he's been turned into a 30-year-old man. Billy Batson finds himself in a similar situation, only in addition to being able to turn back into a kid whenever he wants, he also gets superpowers whenever he's in adult form. Zachary Levi already has a little superhero movie experience under his belt from playing Fandral in the latter two Thor movies, his above comment to Yahoo clearly indicates he's excited to headline Shazam so that he can finally act out his own Big-like story.
Aside from showing how Billy Batson became The World's Mightiest Mortal, no official plot details for Shazam have been revealed yet. Originally the plan was for Shazam to battle Black Adam, but with Dwayne Johnson's iteration of the Kahndaqian anti-hero now leading his own movie, their clash will happen later down the line, and it looks like Shazam will instead deal with Doctor Sivana. Along with Zachary Levi playing Shazam, the movie's main cast includes Asher Angel as the young Billy Batson, Grace Fulton as Mary Broomfield, Jack Dylan Grazer as Freddy Freeman, Ian Chen as Eugene Choi, Jovan Armand as Pedro Peña, Faithe Herman as Darla Dudley and Cooper Andrews as Victor Vasquez.