Deadline reports that Casey Affleck will have a supporting role in his brother Ben Affleck's Whitey Bulger movie. Casey previously starred in Ben's first directorial effort Gone Baby Gone giving a solid performance earning him credibility as a dramatic actor. The trio first worked together on Good Will Hunting which earned Matt and Ben an Oscar for their script. He also co-starred with Matt in the successful Ocean's trilogy directed by Steven Soderbergh. They confirm that Damon will play Whitey Bulger in the film.
Casey's role is unknown but he's recently played dark characters so it would be easy to see him in the role of gangster or lawman. This could mean other actors that Ben has previously worked with in front and behind the camera could join the cast. I'm hoping names like Jeremy Renner are also in contentions for roles.