In an extensive and candid interview with Reelblend Zack Synder talked about the filming process for the reshoots for the HBO Max release of his completed Justice League project. During the interview he talked about his decision to keep the film in full frame as well as the different aspects of the filming he had to integrate due to the Covid restrictions of the film industry.
Two aspects of the film he spoke about extensively was use of digtial cameras and location changes during the reshoots. Initially Zack Synder did not use digital for any of the original filming but had to use them for the reshoots, the Knightmare and Martian Manhunter scenes were filmed digitally unlike the rest of the film.
Speaking candidly, Zack Synder revealed the original scenes with the Martian Manhunter during his visit to Lois Lane and Bruce Wayne were filmed with Green Lantern John Stewart in mind, however he only filmed those scene with the actor during the reshoots. Warner Brothers mandated he could not use a Green Lantern which came to a "loggerheads" where Zack was able to negotiate the Knightmare scene.
He however felt uncomfortable removing a person of color from his film and decided to replace Green Lantern John Stewart with the Martian Manhunter now played by Henry Lennix. It should also be noted that several cast members of color; Kai Zhang who plays Ryan Choi who later becomes the Atom, Karen Bryson who plays Cyborg's mother Elinore Stone, Joe Morton who plays Cyborg's father Silas Stone and of Course Ray Fisher who plays Cyborg were reinstated or had their storylines expanded in Zack Synder's version of the film.
Zack Synder states all of Bruce Wayne's Green Lantern scenes were rendered useless by the mandate as he was covered in Green Light for the original shoot and had to be reshot. Zack Synder had then had to reshoot in his driveway again with Henry Lennix as Martian Manhunter as this was easier during the Covid restrictions the filming was now under.