Story NPCs
I noticed this, too. The AI seems to be very random as to whether or not the NPCs will fight. It doesn’t seem to be proximity based, as with the fight in the first story when you come back to the Pact camp and have to defend against the various minions, I backed all the way into the camp and only a few NPCs actually did anything. It’s frustrating, especially as I found it pretty tough (although I’m willing to attribute at least part of it to testing the Revevant and Reaper, instead of a build I’m very familiar with), and really could have used the NPC support.
But more than that, it… well, it doesn’t quite break immersion for me, but it affects it. It feels weird for the NPCs to be announcing incoming and danger and all that but not move at all. If they don’t want to fight, they could at least cower. :P