I joined Bethesda about two thirds of the way through the development of Oblivion. Not long enough to have an entire system to my name, but enough to have had a small, visible impact.
I implemented various minor AI features for the game, but the one thing that really stood out was that I gave the characters 'shifty eyes' !
The characters in Oblivion would already look at you when you talked to them, and sometimes blink, but never look away. It was creepy. I was asked to add a little bit or randomness in the eyes, and so I took the opportunity to tie their eye movement to their disposition.
If a character was happy, I would animate their eyes up slightly and of course down when sad. When angry I would give them really fast saccades. When scared, they would look left and right. All very simple stuff, especially by today's standards, but it added a tremendous amount of life to the NPCs.