NPC's in Oblivion and Fallout 3 have their own routines. They sleep at night (go in their homes) and wander around in daytime.
They exist for you to talk with, to get mission-objectives, or to let you believe you are not alone (although in Oblivion I always feel alone^^).
NPC's in GTA 4 wander around, or say one liners to other NPC's.
They exist to populate the city and to be splattered.
In fable 2, NPC's are sitting on the ground sometimes, so you can surprise-attack them.
Sometimes they are just standing, doing nothing, waiting for you to come.
Sometimes they jump out of a tree (scripted).
Are these NPC's having a meaningful existence?