The movie will introduce the concept of the DC multiverse, where different versions of its comic book characters could meet up, such as Keaton bumping into Ben Affleck's Bruce Wayne. (Affleck has already signed on.)
Director Andy Muschietti explained how the idea will work during an interview with Vanity Fair back in August.
"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," he said. "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."