As for how director James Gunn got Sylvester Stallone to voice the role of Nanaue/King Shark, it didn't take a whole lot of convincing. Gunn said to him, "I wrote this role for you in The Suicide Squad. It won't take too much of your time." Stallone replied, "Oh yeah?" To which Gunn said, "Yeah. It's a big, kinda chubby, human-eating shark." To which Stallone said, "Anything for you, brother."
James Gunn specifically filled The Suicide Squad with obscure and unknown villains since he felt that was more faithful to the Squad in the comics: "I wanted to stick with John Ostrander's original vision of mostly second-rate antagonists. There's an innate tragic element to supervillains who aren't even that good at being bad."
Joel Kinnaman said the film was an opportunity to give his character a blank slate, and said Flag was sillier, less jaded, more naive, and funnier compared to his portrayal in the first Suicide Squad (2016).
Margot Robbie wears a new costume that features Harley's traditional red and black color palette, with James Gunn taking inspiration from the character's costume in the video game Batman: Arkham City. He wanted her jacket to have "motorcycle gang style"-writing on the back, and chose "Live fast, die clown" over other potential options "Clown AF" and "World's Best Grandpa". Gunn also removed Harley's "Rotten" facial tattoo from previous DC films because both he and Robbie disliked it.
John Cena originally based his portrayal of Peacemaker on George S. Patton, Douglas MacArthur, and R. Lee Ermey, but James Gunn told him Peacemaker was more of a "douchey Captain America" than a real soldier.
Lloyd Kaufman: for a brief second as a patron of the bar the Squad goes to to find The Thinker. Kaufman is best known for his work with Troma Entertainment, where James Gunn got his start writing and directing movies. A quick glimpse of El vengador tóxico (1984), one of Troma's most (in)famous movies, is also featured in the movie.
Pom Klementieff: one of the dancing girls in La Gatita Amable, seen when Task Force X enters the club. Klementieff played Mantis in James Gunn's previous movie Guardianes de la Galaxia Vol. 2 (2017).
James Gunn: [wrestlers-turned-actors in the main cast] In this film, John Cena portrays Christopher Smith/Peacemaker. In Guardianes de la Galaxia (2014), Guardianes de la Galaxia Vol. 2 (2017), and Guardianes de la Galaxia Vol. 3 (2023), Dave Bautista portrays Drax.
James Gunn: [Sean Gunn] James' brother Sean plays Weasel and Calendar Man. Sean also had dual roles in the Guardians of the Galaxy films, where he played Kraglin, while also performing motion capture for Rocket.