Michael Keaton's original Batman suit was infamously restrictive, forcing him to move his entire body to look around. For this movie, the costume designers created a more flexible suit that allowed for greater range of motion, but when Keaton first tried it on, he insisted it be made stiffer.
Michael Keaton was 71 at the time of this film's release, making him the oldest actor to play Batman in a live-action film or TV series. The only actors who were older were Adam West, who was 88 when he voiced Batman for a final time in Batman vs. Two-Face (2017) (released after his death), and Olan Soule, who was 74 when he voiced Batman for a final time in the seventh season of Super Friends (1973).
Director Andy Muschietti described Michael Keaton's reaction to the Batcave set and Batman suit, "He stayed like this [eyes wide] for a while. I didn't want to interrupt him. I just wanted for him to take it in. Who knows what was going on there? But something was going on there... It was funny because at one point during the scene where we shot him in the full suit, he was like, 'Can you take a picture? It's for my grandson.' It was one of those moments where he really showed something was inside that was very emotional."
Michael Shannon was hesitant to return as General Zod because he was "very upset" with the circumstances of Zack Snyder's exit from the franchise. He finally agreed after Snyder gave him his blessing.
In contrast to the infamously troubled production of Justice League (2017), Ben Affleck said that he had a great time reprising his role of Bruce Wayne/Batman in this film.