Harry Lennix played Calvin Swanwick in Zack Snyder's Man of Steel and Batman v Superman: Dawn of Justice, and although it wasn't known to the actor at the time, it was Snyder's intention that the character would be revealed to be Martian Manhunter in Justice League. Obviously that didn't pan out and Lennix didn't appear in Justice League, but the actor will get to reprise the role for the upcoming Snyder Cut of the film.
While speaking with Collider, Harry Lennix confirmed that he wasn't aware of Swanwick's true nature for some time, and although he would have been revealed as Martin Manhunter in Justice League, he never got the chance to read the script. "I never saw the script," Snyder said. "I didn't get a chance to read the script, because… they were still being developed, and they keep all of that under lock and key." Lennix's role on The Blacklist kept him from shooting on Justice League and Snyder had already left the project by the time he became available, but Lennix revealed that he returned to shoot new footage for the Snyder Cut of Justice League last October and complete the grand design. In terms of how much Martian Manhunter we'll get in the new cut, Lennix described the shoot as a "small little thing" taking place on primarily on green-screen.
We still don't know when Zack Snyder's Justice League will drop on HBO Max, but Snyder teased last month that it's likely that the project will be rated R. "Here's one piece of information nobody knows: The movie is insane and so epic and is probably rated R — that's one thing I think will happen, that it will be an R-rated version, for sure," Snyder told Entertainment Weekly. "We haven't heard from the MPAA, but that's my gut." As far as what might give Justice League an R-rating, Zach Snyder explained, "There's one scene where Batman drops an F-bomb. Cyborg is not too happy with what's going on with his life before he meets the Justice League, and he tends to speak his mind. And Steppenwolf is pretty much just hacking people in half. So [the rating would be due to] violence and profanity, probably both." When Snyder released his director's cut of Batman v. Superman: Dawn of Justice, it was also given an R-rating due to additional violence, so an R-rated Justice League isn't a huge surprise.