• David Corenswet as Clark Kent / Superman: A survivor from the destroyed planet Krypton who lives on Earth as a superhero. He hides his identity at his day job, working as a journalist for the Daily Planet in Metropolis. Director James Gunn said the film's version of Superman would be around 25 years old, making him more established than Tom Welling's version from the series Smallville (2001–2011) but younger than Henry Cavill's DC Extended Universe (DCEU) version. Producer Peter Safran described the film's version of Superman as "the embodiment of truth, justice and the American way; he's kindness in a world that thinks of kindness as old fashioned".
• Rachel Brosnahan as Lois Lane: A reporter for the Daily Planet who is a close colleague and love interest of Clark. Brosnahan described Lois as "fiercely intelligent" and feisty. To prepare for the role, Brosnahan spoke to a handful of "really brilliant journalists" to help her understand the mindset that people who work in newsrooms and broadcasts have, which her take on Lois doesn't have but she deemed their suggestions still helpful due to how generous with their time and fantastic they were.
• Nicholas Hoult as Lex Luthor: The CEO of LuthorCorp and the arch-nemesis of Superman. Hoult was inspired by Michael Rosenbaum's portrayal of Lex Luthor in the series Smallville, choosing to work out for the role based on the depiction of Luthor in the comic book All-Star Superman (2005–2008) by Grant Morrison and Frank Quitely. His son Joaquin shaved Hoult's head in preparation for the role. Gunn took inspiration for the character from Brian Azzarello's limited comic book series Lex Luthor: Man of Steel (2005) as well as Jerry Siegel's "reckless scientific genius" version of Luthor in comic books from the 1950s and 1960s. Hoult describes his take on Luthor as hating Superman for having the ultimate power and ability to do everything but not aligning with Luthor's own beliefs about what's right.
• Edi Gathegi as Michael Holt / Mister Terrific: A superhero and inventor who uses a variety of high-tech gadgets to fight crime
• Anthony Carrigan as Rex Mason / Metamorpho: An archaeologist and superhero who can "transmute elements in his body into various forms". Carrigan said it was refreshing to play a superhero after previously portraying the Batman villain Victor Zsasz in the television series Gotham (2014–2019). He felt that he could bring some "authenticity and truth" to the role by comparing Metamorpho's concerns about his abilities to Carrigan's own alopecia.
• Nathan Fillion as Guy Gardner / Green Lantern: An abrasive galactic peacekeeper in the Green Lantern Corps. The film retains the character's "iconic bowl cut haircut" from the comics.

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Warner Bros. Pictures was planning in 2012 for the film Man of Steel (2013), based on the DC Comics character Superman, to start a shared universe, which became known as the "DC Extended Universe" (DCEU). They announced a full slate of DC films in October 2014, prioritizing Batman v Superman: Dawn of Justice (2016) as the second DCEU film after Man of Steel failed to meet the studio's financial expectations. Despite this, they said an undated sequel to Man of Steel was in development with Henry Cavill set to reprise his role as Clark Kent / Superman. Man of Steel director Zack Snyder said they had been considering the imprisoned Kryptonians from the first film and the character Brainiac as antagonists for the sequel before work began on Batman v Superman. A Man of Steel sequel was in active development as a top priority for the studio in August 2016, which Cavill's manager Dany Garcia confirmed the following month. Amy Adams, who portrayed Lois Lane in Man of Steel, said in November that the studio was working on the screenplay. Matthew Vaughn was Warner Bros.' top choice to direct the film and had preliminary conversations about it by March 2017. Vaughn and comic book writer Mark Millar had previously pitched a new Superman trilogy prior to the development of Man of Steel, in which the destruction of the planet Krypton would not occur until after Superman had already grown-up on the planet. After the troubled production of the DCEU film Justice League (2017), Warner Bros. re-thought its approach to DC projects. By the end of 2017, a Man of Steel sequel was not coming "anytime soon, if at all". Justice League producer Charles Roven said story ideas had been discussed but there was no script.

Before the release of Mission: Impossible – Fallout in July 2018, director Christopher McQuarrie and co-star Cavill pitched their take on a new Superman film, but Warner Bros. did not pursue the idea. Later in 2018, the studio asked James Gunn to write and direct a Superman film, but he was uncertain if he wanted to take on the character. He did not have a clear vision for such a film, especially since Superman was a well-known character unlike the Guardians of the Galaxy who he had adapted for Marvel Studios in the film Guardians of the Galaxy (2014). Gunn chose to make The Suicide Squad (2021) instead. In September, negotiations for Cavill to reprise his role for a cameo appearance in Shazam! (2019) ended due to contract issues, as well as a scheduling conflict with Cavill's Fallout commitments. The actor was reported to be parting ways with the studio, with no plans for him to reprise his role in future projects, but Cavill said in November 2019 that he had not given up on the character and still wanted to do the role justice. At that time, Warner Bros. was unsure which direction to take the character and was talking to "high-profile talent" about the property, including J. J. Abrams—whose company Bad Robot signed an …

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In December 2023, Gunn said most of the film's score and major themes had been written, but the composer had not been announced because their deal had not been finalized. In February 2024, he announced that John Murphy was composing the score after doing so for The Suicide Squad, The Guardians of the Galaxy Holiday Special (2022), and Guardians of the Galaxy Vol. 3. In December, Gunn confirmed that Murphy was using a new version of John Williams's "Superman March" theme from the 1978 Superman film. Gunn considered Williams's score to be one of the greatest of all time and had associated his theme with Superman from the beginning of development on this film. He saw the theme's inclusion as one way for the film to harken back to past iterations. The film does not have as many licensed songs as Gunn's previous superhero projects.