Summary
- James Gunn didn't choose Michael Rosenbaum to play Lex Luthor in Superman: Legacy, and he shared the reason why.
- Nicholas Hoult has been officially cast as Lex in the DC Universe movie.
- Gunn revealed his preference for Rosenbaum as his "favorite Lex" so far.
James Gunn did not cast his favorite actor to play Lex Luthor as the character in Superman: Legacy's cast also includes multiple DCU heroes, as well as leads David Corenswet as Superman and Rachel Brosnahan as Lois Lane.
On Threads, Gunn revealed why he did not cast Smallville's Michael Rosenbaum, his "favorite Lex," as Superman: Legacy's Lex Luthor.
According to the latest Superman: Legacy update by Gunn, the choice was due to the director wanting "a Lex who was contemporaries with Superman." Despite having enormous experience as Luthor due to having played the villain over seven seasons of Smallville, Rosenbaum is 51. At 34, Hoult is in the same age range as Corenswet, who is 30, explaining the casting.

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Could Michael Rosenbaum Ever Play Lex Luthor Again?
A Smallville Revival Might Be On The Way
While he might not be the main Lex Luthor of Gunn's new DC Universe, there are still a few ways for Rosenbaum to come back as the villain in the future. Throughout his time on Smallville, the actor delivered one of the most nuanced performances in any superhero project, making Rosenbaum the best live-action Lex Luthor so far, which explains why he is Gunn's favorite. Lex went down a tragic road to become the iconic villain he is in the comics in the series.
The multiverse is a popular aspect of current superhero movies and series, and it could allow Rosenbaum to finally play Lex Luthor again. While he will not play Lex in Superman: Legacy, Rosenbaum could appear as his version of Luthor from Smallville in a future version of Crisis on Infinite Earths in the DCU. The DC TV's Arrowverse already did its own version of the event, which brought back Smallville's Clark Kent, Tom Welling, setting a possible path for Rosenbaum's return in a movie version of Crisis.
More likely than a live-action return as Lex, however, is Rosenbaum reprising his Smallville villain for an animated revival of the DC series. Rosenbaum and Welling have talked multiple times about how they are developing a Smallville animated series that aims to bring back most of the show's cast to continue from where the show left off. A Smallville sequel series where Rosenbaum's Lex Luthor is the President of the United States and goes up against Welling's full-fledged Superman is a better use of the Lex Luthor actor than playing a new version of the villain in Superman: Legacy would have been.
Source: James Gunn/Threads