“People are much more difficult to simulate than rocks and trees, not just because of relative complexity, but because we’re more wired to scrutinize our fellow humans” (Artificial Intelligence In The Uncanny Valley, 2007).
In the original context this was talking about graphics, but it applies equally to text based interactions with NPCs. On thing we are wired to look for is emotion; a character who never reacts to events or varies their emotional state seems a lot less real.
Continuing from part one, here are some more ways emotions work in real life that have potential for IF.