I've got Rick first here, because, well, it almost goes without saying. There is absolutely no way The Walking Dead is going to kill off its leading man in Season 6, if ever. Andrew Lincoln, in many ways, is the show. He may not be the best character - it tends to be up and down with Rick, generally depending on his state of mind - but he's the leader of the group, and has been since the very first episode. Lincoln, meanwhile, is a strong actor, probably the most recognisable, and has been the face of the marketing, again, since inception. It'd be interesting to see how this show worked without Rick, but I think the short answer is that it wouldn't. It needs him holding everything - and everyone - together, and if it does ever kill him off, it'll be way past Season 6.