Just what it says it is. Who's joining the show, who might be recast.
No untagged spoilers beyond book/season one please.
Example: If Arya is recast to show that the character has grown or years have passed, fine. If she's recast because the actress didn't want to sign up for only a few episodes because her character was getting killed off, don't tell us. Let folks find out when the episode airs.