I apologize if this was answered somewhere; I tried searching, but didn't come across it.
In most of the quests I can think of in the base game and other expansions, when you have NPCs (especially ones that can be attacked), they generally say they can/cannot be affected by conditions and/or that they automatically pass/fail attribute tests.
However, I've noticed some of the quests don't. One, for instance, is the Shadow of Nerekhall quest "The True Enemy", where the guards are treated like hero figures, and can attack and be attacked. However, for the guards, there is no clarification (that I saw) as to how conditions affect them.
Stun, Immobilize, and other test-less conditions are easy. Even Weakened, which requires a "rest" action to undo, isn't too hard to interpret - they just can't get rid of it. However, what about tests like Poisoned or Diseased, which require an attribute test. Do they automatically pass? Fail? What about other tests (like if a monster ability requires a test on the target or something).
For our game, we just assumed they automatically failed, and it didn't seem to be TOO bad, but if I recall, reading through the quest guide, there were a few quests where the NPC's relationship to conditions/attribute tests is "unknown".
Maybe there's a catch-all rule covering that somewhere that I overlooked, though.