DC fans got some pretty unexpected news on Thursday, with confirmation that both Ben Affleck and Michael Keaton will be reprising their roles as Batman in the upcoming The Flash movie. While both actors' version of the Caped Crusader had previously been tied to the Ezra Miller-led film in one way or another, the confirmation from director Andy Muschietti proves that it's definitely going to happen. If this news has left you wondering exactly how both Affleck and Keaton's Batmen will appear in the film - despite existing in wildly different versions of continuity - there is established precedent in it, thanks to The CW's "Crisis on Infinite Earths" crossover.
The event, which aired across December and January on the network, saw the show's roster of Arrowverse shows adapt the "Crisis" comic miniseries of the same name, which followed the destruction and recreation of the DC multiverse. In the television adaptation, this involved weaving in cameos and Easter eggs to a menagerie of different "Earths", many of which were based on existing DC Comics adaptations. One of the first "Earths" shown was Earth-89, which featured the return of Alexander Knox (Robert Wuhl), a Gotham Globe reporter and ally of Keaton's Bruce Wayne. Knox sat on a bench reading a newspaper that read "Batman Captures Joker", only to look up and notice the Bat-signal shining on the growing red skies above. Earth-89 was then seemingly destroyed by a wave of antimatter -- but didn't seem to completely be down for the count.
The fourth hour of "Crisis" proceeded to link Affleck's Batman to the rest of the DC Multiverse in a roundabout way, with a completely unexpected cameo from Miller's version of The Flash. In a quick sequence, Miller and Grant Gustin's version of The Flash met face-to-face, and quickly became baffled by the fact that they were both Barry Allen. Gustin's Barry even seemed to unintentionally give Miller's Barry the idea to call himself "The Flash", before Miller's Barry disappears, remarking that he needs to tell Victor Stone/Cyborg (Ray Fisher) about this.
By having Miller and Gustin canonically share the screen together, "Crisis" firmly established Miller's fellow Justice League members - including Affleck's Batman - exist within one extension of the DC multiverse. While the Earth the main DC Films characters are on doesn't yet have a proper label (although, based off of comic precedent, they could probably go by "Earth-1", now that the Arrowverse's main Earth goes by "Earth-Prime"), it still firmly proved that there's a connection between the two -- and, by extension, a connection between that Earth and Keaton's Earth-89.
"This movie is a bit of a hinge in the sense that it presents a story that implies a unified universe where all the cinematic iterations that we've seen before are valid," Muschietti said when confirming Affleck and Keaton's roles. "It's inclusive in the sense that it is saying all that you've seen exists, and everything that you will see exists, in the same unified multiverse."