Here is the news from The Talking Dead main website. It doesn't mention who the mystery guest is. But it does mention that there going to be a mystery cast member guest. http://blogs.amctv.com/the-walking-dead/talkingdeadcom/ We don't know who that mystery guest is. It could be a past cast member, a present cast member or a future cast member. But it will be someone that will be involve in episode 13(Forget).