When zombies (or walkers or however you refer to them) are in an easily kill-able location, such as the fence at the prison, there are many times where spears or swords or other lengthy objects are thrust into and through a zombie's head. You can see the actor push it deep into the head, you can see the pointy end emerge from the back, and you can also pretty clearly (IMO) see the CGI gore fly out of the back.
I get the gore. It's pretty obvious. But, how are the actors thrusting these lengthy objects into the actors portraying zombies? Is it all hidden by camera angles? I can't see the spears/swords being CGI, that'd be too tricky.