My bad. I probably should have worded it better. I'm assuming these people haven't just been cast recently. I took it for granted they were spacing out the announcements to keep up people's excitement and interest. They've been filming for ages now, they must have done the casting before they started. I don't really follow film that much but I'm pretty sure they wouldn't be casting as they go along. That would be weird, wouldn't it?
We know there are some really significant figures we still haven't heard about. It's gotta be a 'keep up the excitement' thing, I'm sure of it.