'Zack Snyder's Justice League' has a cameo fans have been waiting years to see. Here's how it came together.

• Warning: There are massive spoilers ahead for "Zack Snyder's Justice League."
• Martian Manhunter (Harry Lennix) appears twice in Snyder's four-hour cut of the film.
• Harry Lennix tells Insider how a text from Snyder brought him back into the DC universe.

Zack Snyder's campaign to "unite the seven" has finally happened, six years later.

In addition to Batman, Superman, Cyborg, Wonder Woman, the Flash, and Aquaman, "Zack Snyder's Justice League" introduces Martian Manhunter, a DC superhero fans have longed to see in Snyder's finished cut of the film.

Scrapped from the 2017 theatrical cut, the movie finally reveals Harry Lennix's General Calvin Swanwick (from 2013's "Man of Steel") as J'onn J'onzz, the hero from Mars in not one, but two scenes.

He first reveals his alter-ego after a touching encounter with Lois Lane (Amy Adams), where he tells her "The Daily Planet," and the world, need her as much as they needed Superman. He appears again at the film's end, revealing himself to Bruce Wayne as someone who wants to help the Justice League defend Earth from Darkseid.

"Zack, from time to time, will send me a text," Lennix told Insider by phone on Tuesday. "I think this one basically said, 'Hey, do you want to come and play?' I said, 'Anytime.'"

Snyder announced via social media app Vero in May 2020 that he would release his original vision for the 2017 movie on HBO Max this year. Lennix told Insider he learned the "Snyder Cut" was happening when everyone else did.

"I had no idea. I did not know anything beforehand," Lennix said. "My immediate reaction, and I put this on my social media, I said, 'Things don't just happen. Things happen just.'"

"This was, in some way, even a surprise to Zack, that somebody would have said, 'Yes, you can make a movie,'" he continued, explaining that he sees the "Snyder Cut" as a "symphony of styles and myths."

Lennix said he wasn't aware there was some other version of the film until Jason Momoa said he saw the then-fabled "Snyder Cut" in August 2019. Ben Affleck and Gal Gadot then threw their support behind it in November 2019 on the two-year anniversary of the theatrical release.

"I didn't have any kind of indication that it existed until somebody in the movie, one of the 'Justice League' [members], actually sort of sneaked that out there, but I'm glad he did," said Lennix.

Returning to play Martian Manhunter was the happiest surprise in Lennix's career

Lennix's General Swanwick was supposed to be in the original theatrical cut. Some original posters for the 2017 film had always teased seven heroes instead of six, hence the "unite the seven" movie campaign.

A studio mandate from Warner Bros. to shorten the film changed that.

"I was all set to go to London to start shooting on the movie back in 2016," Lennix said, adding that the studio's demand to make "Justice League" two hours originally cut him out of the movie: "My character got lost in the shuffle."

If it weren't for HBO Max stepping in, Lennix said he wouldn't be in the movie at all. He didn't even see his finished character on-screen until watching the final cut of the film with his wife. He loved it, saying, "I wish I could look like that all the time. I think it's beautiful."

The road to Lennix playing a superhero was a long one. The actor didn't know the plans for him to play a superhero when he first joined 2013's "Man of Steel." In fact, he still didn't know Snyder's idea to make him J'onn J'onzz until he returned to work on the "Snyder Cut."

Becoming Martian Manhunter was "a wonderful surprise."

The small moment in 'Man of Steel' that hinted at Martian Manhunter years ago

Though Lennix wasn't aware at the time, Snyder always appeared to have Lennix in the back of his mind to play a larger role - even during "Man of Steel."

Lennix recalled a scene that they filmed for the movie which hints that General Swanwick was, in fact, the iconic hero from Mars all along.

The moment, which takes place one-hour and 12 minutes into the film, occurs when Clark Kent/Superman is taken in by the US Air Force. High-ranking military figures watch him interact with Lois Lane in an interrogation room behind a glass panel. While Superman isn't supposed to be able to see them, he can because of his many powers.

When Superman rises from the interrogation table and snaps off his handcuffs, it causes the military figures to back away from the glass, except for Swanwick, who steps forward.

"I wish I could say that I had an inkling of it, but my belief is that the 'Man of Steel' piece where I am standing opposite Superman on the other side of that two-way glass ... I think it was at that moment that I asked Zack, 'Should I actually hold my ground here or back away from the glass, like everybody else is?'" said Lennix.

"He, at that point said, 'No, I think you stand your ground. I even think you step forward a little.' And, I think it was in that conversation that the idea may have actually taken root in his mind," Lennix said of making General Swanwick the Martian Manhunter.

When you watch the scene now, it plays as if Swanwick might have a superhuman power because he knows he can go toe-to-toe with Superman.

In 2019, Snyder shared his original storyboards for the Martha Kent and Lois Lane scene, seen in 2021's "Snyder Cut," which would have revealed Lennix as the superhero on Vero.

Lennix shared the storyboard drawing of himself as Martian Manhunter on Twitter in January, celebrating that he would play the hero.

Lennix filmed his new scenes for 'Zack Snyder's Justice League' in August and October 2020

Technically, there's only one completely new scene that was filmed for the "Snyder Cut," which was Bruce Wayne's dream where Batman and the Joker speak with one another for the first time.

But Lennix also filmed scenes so he could be put into the new version of the movie.

"I did most of my work off of a green screen," Lennix said of the Martha Kent/Bruce Wayne scenes. "I did two-part days. There was one day I went in August and did the scene at the elevator and then I did the other scene with Ben Affleck. I did that sometime in October."

Lennix said he shot the scene on green screen in a studio in New York. At the time, he was back on set of "The Blacklist," which is currently filming.

"They just put me in, digitally," Lennix said of how the final moments of the film came together. "It's pretty, pretty amazing what they can do now."

Lennix doesn't know what Martian Manhunter's future character arc is, but thinks it's possible we could see more

"I don't know what the plans may be for the character," Lennix said when asked if Snyder told him anything about his character arc.

An official exhibit for the film in Dallas, Texas, which opened in March, has Snyder's plans for two "Justice League" sequels on display.

"I have not gone down to Dallas to check it out," Lennix said. "My wife's family is down in that part of the country. I would love to know just because I'm a big fan of the character now, even though I knew not much about him, and I'm a huge fan of Zack."

The "Snyder Cut" ends on a massive cliff-hanger with the arrival of Martian Manhunter making himself known to the Justice League, warranting a sequel. Lennix says he thinks the fans have a good chance of getting a sequel off the ground.

"I think the fans have proven that they have a lot of power," Lennix said when asked his thoughts on a new hashtag, #RestoretheSnyderVerse, that fans have started using to demand more of Snyder's vision.

"I think the fans can absolutely move mountains," Lennix added. "And why not this one? This one would actually be a fairly easy lift."

"Zack Snyder's Justice League" is streaming now on HBO Max. You can read our review here.