This is actress Danai Gurira's first time on screen as Michonne. The character of Michonne was at the end of the season 2 finale, but she did not portray since she had not been cast at the time. An extra filled in for the spot of Michonne in that episode
The method Daryl uses to kill the prison guard walker by pushing his helmet forward and driving his knife into the base of the walkers skull is one of the sentry neutralization technique taught by the U.S. Army.
The episode starts off with a closeup view of an eye and then pans out to show a walkers face. The finale episode for this season starts out the same way, only when the camera pans out it reveals the Governor's face. Possibly a metaphor for how the season starts off with walkers being the biggest danger to the group and ends with them learning that other people are often a greater threat to them.