An extensive article has been done over at THR on how actor Ben Affleck was cast in the role of Batman/Bruce Wayne in the upcoming "Man of Steel" sequel at Warner Bros. Pictures.
The article indicates several other actors were approached before Affleck, including Josh Brolin and Ryan Gosling, but they passed. Snyder reached out to Affleck to gauge the star’s interest earlier this year and actor was curious.
Once he was satisfied with how the story would unfold and how the character would be portrayed, negotiations began in secret - to the point that many in both Warners and Affleck's agency WME were unaware. Affleck has reportedly signed for multiple films, which would likely include a "Justice League" film.
Afflecke will next shoot "Gone Girl" for David Fincher from September to February, then film the Batman vs. Superman project from February to August. During the filming he will be able to prep for his "Live by Night" adaptation which will go into production next Fall, immediately after Snyder's film wraps.