I find that my NPCs often react rather than respond, by this definition, especially at first. I think that in order to establish character I want NPCs to really telegraph several defining features until characters begin to recognize them. Since reactions tend to involve gut reactions based on internal attitudes, these are really useful to help broadcast NPCs’ internal selves to my PCs. That said, I need to remind myself that my NPCs can think and not react rashly to everything the PCs do.