Zack Snyder's Justice League is set to introduce not just all the major DCEU superheroes so far, but a new one as well, namely J'onn J'onzz, the shape-shifting superhero known as the Martian Manhunter. Snyder has previously confirmed that the Manhunter has existed in the DCEU ever since Man of Steel, in the guise of General Swanwick, played by Harry Lennix. In an interview with Variety, Lennix confirmed that the scene in Man of Steel in which a handcuffed Superman confronts a group of soldiers led by Swanwick is what birthed the possibility of his character being the Manhunter.

"I think to some extent, that is what has sort of started the speculation [about Martian Manhunter], that Swanwick didn't shrink away after Superman rather nonchalantly takes his handcuffs off and walks up to the window. This is now getting on to about nine years ago that we shot that scene. I think I asked Zack the question, "Am I going to shrink back here?" Because I'm a general, I've obviously faced danger. And, he said, "No, in fact, you stand your ground, maybe even you move forward a little." Perhaps that's what got those wheels turning, that at least gave the suggestion that that might be a plausible reveal."
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It seems Swanwick being J'onn J'onzz was not a possibility that was set in stone during Man of Steel, but the team behind the movie recognized the potential after fans reacted to the scene of Superman confronting Swanwick. Thus, Lennix reprised his role as Swanwick in Batman v. Superman, and in 2019, Snyder shared a storyboard idea with fans from Zack Snyder's Justice League which would have shown Swanwick transforming into the Martian Manhunter. According to Lennix, that was when he realized his character was definitely a superhero.

"I'm pretty sure that's exactly what I found out. I don't think there was any sort of advanced word. There are a lot of people who intuited it somehow, like fans bouncing that idea around. So I don't know if somebody knew about it outside of Zack before that. But I did not know until I saw that storyboard... From time to time, you know, Zack will text me things or email me things. I've certainly done projects with him, such as "Army of the Dead," and I knew that was coming. But he likes to surprise me, and I think I'm not unique in that way. But, you know, there was no time that I could have found out about it that would have been a happier time."

Zack Snyder's Justice League stars Ben Affleck as Batman, Gal Gadot as Wonder Woman, Henry Cavill as Superman, Amy Adams as Lois Lane, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, Ciarán Hinds as Steppenwolf, Jesse Eisenberg as Lex Luthor and J.K. Simmons as Commissioner Gordon. The film premieres exclusively on HBO Max on March 18. Variety was the first to bring these words.