We've already heard from Chuck's creator, Josh Schwartz, that the last half of this season is going to have some game changing episodes. Now, Schwartz has hinted to Kristin Dos Santos a little more about what that means.
According to Schwartz, Chuck still doesn't know that Sarah was trying to save Chuck's life when she killed a man in front of him. Hey says "that's a dynamic that obviously is going to be complicated."
He then adds that "there's a very sexy MI6 agent who comes in to help on a mission for a couple of episodes, played by Jonathan Cake. He's a real sort of badass Gerard Butler - type who definitely catches Sarah's eye. There are a couple things that are going to happen towards the end of this season that are sort of a point of no return for Sarah and Chuck."