Actors Mark Hamill (Joker) and Kevin Conroy (Batman) have both played these characters for over 25 years. Both actors have repeatedly said that they were "retired" from voicing the characters. Hamill has said that while he enjoyed every minute of being the Joker, the years of voice work had put a huge strain on his voice and that someone else should have the opportunity to "don the psychopathic clown". Both Hamill and Conroy have stated that at their age the repetitive nature of the characters had become tiring. Both did say if there were ever a great story to reprise each character, they'd return. In addition to this series each returned for Batman: Arkham Knight and the animated film, Batman: The Killing Joke.
While most notably Kevin Conroy and Mark Hamill reprise their iconic roles as Batman and The Joker, respectively, there are a number of other voice actors who reprise their roles from previous DC productions. Kevin Conroy/Batman, Mark Hamill/Joker, Khary Payton/Cyborg, Charlie Schlatter/Flash, Lacey Chabert/Zatanna, Tara Strong/Harley Quinn, Sean Astin/Shazam, Gilbert Gottfried/Mr. Mxyzptlk, Fred Tatasciore/Solomon Grundy, David Sobolov/Gorilla Grodd, Max Mittelman/Jimmy Olsen, Jake T. Austin/Blue Beetle, Julianne Grossman/Hippolyta, Christian Slater/Deadshot,
The series was originally conceived as a solo Batman project until series producers Alan Burnett and Butch Lukic were told to make it a Justice League series. This was done to promote the 2017 Justice League movie.
The first 4 episodes of the series, which combine into the TV movie "Shazam Slam", are the only episodes to premiere in prime time. The rest of the series would mainly air on Saturday mornings.
Although credited as "Billy Batson / Shazam" the character is never addressed on-screen by name. This is likely because of potential confusion for casual viewers who are not comic book readers. The character was originally named Captain Marvel and debuted in comics in 1940. Marvel Comics introduced a character called Captain Marvel in 1967 and over the years has created several other characters also called Captain Marvel. D.C. Comics eventually changed the character's name to Shazam. Similarly, the Wizard is never addressed by name (except in the first double episode, the wizard says to Billy Batson to call out my name to receive your powers to which Billy yells out SHAZAM & later Batman refers to him as the wizard Shazam.) . That character was originally named Shazam and saying his name transformed Billy Batson into Captain Marvel. In-universe the character died of extreme old age.