Ben Affleck's Batman will return in The Flash movie, and announced that sent the DC fanbase into a wave of elation before the DC FanDome event. It's not just the fact that Ben Affleck will be making a return to the Batman role that fans loved him in, it's that he will be sharing the role with Michael Keaton's Batman, in a one-of-kind story about The Flash going through a Back to the Future-style journey through time and alternate realities. This appearance from multiple Batmen may be confusing to a lot of mainstream moviegoers - but there's strong DC Multiverse history that explains it.
In the 2011 crossover "Flashpoint," Barry Allen/The Flash inadvertently creates an alternate DC Comics timeline that's twistedly different than the one we know. Barry causes the disruption to reality by running back in time to prevent his mother's tragic murder; unfortunately, the ripple effects prevent the DC superheroes from ever coming together, leaving the world on the brink of annihilation as a result. The Flash movie makers (including director Andy Mushcietti) have already confirmed that the film will be set in a Flashpoint framework, with Barry creating an alternate timeline of the DC movie universe.
Flash: A Tale of Two Batmans
A big point of Flashpoint is a sub-plot about Barry's interaction with Batman - two versions of Batman, in fact. The first Batman he meets is a much more violent, gun-toting version of the hero, who turns out to be Thomas Wayne (as his son Bruce was the one killed in the alley). Thomas mentors Barry and helps him both try to avert global annihilation and get back to his own timeline. In the end, Barry does make it back to a (slightly altered) version of the main DC Universe, where he delivers a final emotional punch to the story: A letter from Thomas Wayne, to Bruce. Flashpoint actually ends with Batman actually shedding tears (albeit off-panel).
How Ben Affleck & Michael Keaton Fit Into The Flash
In that sense, The Flash movie clearly has a framework to do much the same as its comic book predecessor. Andy Mushietti confirmed to Vanity Fair that Affleck's Batman is "a very substantial part of the emotional impact of the movie. The interaction and relationship between Barry and Affleck’s Wayne will bring an emotional level that we haven’t seen before... He’s the baseline. He’s part of that unaltered state before we jump into Barry’s adventure."
Affleck's Batman and Ezra Miller's Flash have a mentor/mentee relationship that goes back to Justice League, and Muschietti points out the loss of their mothers will be an emotional connection that helps galvanize the film's events. That makes it seem like Affleck's Batman could indeed be the bookend to The Flash movie - first inspiring Barry's rebellious act to alter time, and then serving as the sign of the "normal" world that Barry comes back to, at the end. Meanwhile, Keaton would be the Batman that mentors Barry in the alternate world, with all the changes that age and a different set of experiences have caused.
What's nice about that presumed setup is that they also open the door for some nice winking meta subtext to both Affleck and Keaton returning. Both men have had their struggles with the Batman role, and the legacy that always follows after. It would be nice closure for both actors to bring some of those real-life pains into their respective performances (since we'll all be thinking about it anyway). In the same sense, that catharsis could end up opening some new doors...
The Flash will be released in 2022. We expect to lean more about it during the DC FanDome event.