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Eisenberg had originally met with Zack Snyder about a different role in the movie. Snyder said he had been meeting with “the usual suspects that you’d imagine for Lex”… meaning someone older. Warner Bros. was interested in Joaquin Phoenix, for example. But once Eisenberg was cast as Luthor, screenwriter Chris Terrio tweaked the script to better suit a younger version of the character.
From Batman News: