I’m surprised at just how many opportunities the game allows the St. Johns to prematurely blow their cover (or, at the very least, raise some suspicion).
There’s an opportunity right when you arrive at the dairy, before you even head out to secure the perimeter with Mark. If you try to go through the gate leading to the barn, Andy immediately stops working on the generator and stands up to defensively ask Lee exactly why he wants to check out the barn. Regardless of what Lee replies, Andy assures Lee that there’s “plenty of time to check out the barn later” and keeps urging him to secure the perimeter with Mark instead.
According to Andy’s voice actor, Adam Harrington, Andy wanted to get rid of Mark & Lee from the get-go, as he saw them as potential threats to the St. Johns’ plans of turning the group into trade meat. I don’t think Andy planned on the bandit attack, and he probably didn’t even plan on Lee & Mark volunteering to go ahead without him in the first place, so perhaps he planned on getting rid of them himself while the three of them were out there, make it look like an accident or something. Then, once Lee assured him that he and Mark could do it on their own, Andy formulated a new plan: he’d electrocute them instead, hence him turning on the fence despite knowing they were going to be out there touching it (Andy claims he turned the fence on because he heard Mark yell and mistook that for an “all clear!” signal; however, Mark didn’t yell until after he was shot with an arrow, at which point the fences had already been turned on). Andy gives a confused and somewhat disappointed look at the men as they return before quickly switching to pseudo-shock and concern.
I just love how Telltale put so much thought into the little details and mannerisms of each character that would give subtle hints to the St. Johns’ horrible secret before it was actually revealed.