Let's start with an easy one. Viola Davis reprises her role as Amanda Waller across the DC universe.
Amanda Waller was teased in the film's trailers. Warner Bros.
Waller has become one of the few loose threads keeping the DC Universe connected when it may feel rather disparate.
In "Black Adam," the government official and shady leader of Task Force X (aka the Suicide Squad) recruits a new group, the Justice Society of America to rein in Black Adam once he awakens from a near 5,000-year slumber.
Waller claims it's for the world's protection. As we've come to know from "The Suicide Squad," Waller isn't usually sharing every side of the story, so she likely has her own personal agenda for keeping Black Adam under wraps from the rest of the world.
In "Black Adam," we learn that Waller seems to have just about every DC hero and villain on speed dial.
Djimon Hounsou very briefly reprises his role as the wizard, Shazam.
Djimon Hounsou in "Shazam!" Warner Bros.
It's easy to miss Hounsou, but he can briefly be seen along with the other wizards bestowing Black Adam's son with the powers of the gods.
Henry Winkler appears as the original Atom Smasher.
Henry Winkler's appearance in the film was the biggest surprise that didn't get out in advance. Alberto E. Rodriguez/Getty Images
Al Rothstein (Noah Centineo) takes on the mantle of Atom Smasher in "Black Adam."
Early in the film, Al's seen on a video call with his Uncle Al, played by Winkler, who we learn was the original version of the hero. Young Al is taking over superhero duties from his uncle.
Jennifer Holland makes a surprise appearance as Task Force X member Emilia Harcourt.
Jennifer Holland made her debut as Emilia Harcourt in "The Suicide Squad." DC / Warner Bros.
Harcourt previously appeared in "The Suicide Squad" and "Peacemaker." In "Black Adam," she's shown working at a secret base for Amanda Waller's Task Force X where a number of DC characters are being held captive.
Harcourt helps take in Black Adam when he decides to relinquish his powers.
Henry Cavill reprises his role as Superman in a surprising mid-credits scene.
Henry Cavill as Superman in "Man of Steel." Warner Bros.
Aside from "Zack Snyder's Justice League," Cavill hasn't been seen in the DC universe since 2017's "Justice League."
He appears in a shocking mid-credits scene where he introduces himself to Dwayne Johnson's Black Adam and tells him they should talk.
According to The Hollywood Reporter, the Rock pushed to bring Cavill back as the Man of Steel, even going over the head of former DC Films' head Walter Hamada to make it happen because he believed that was what fans wanted. Johnson has been immensely vocal about wanting to go toe-to-toe with Cavill's Superman in a future film.
You can read our explainer on his return here.