Non-player characters are very important when creating this kind of world. BioWare can get away with having everyone stand around forever, but in an open world, the people must be moving and acting. It’s surprising how many games fail at this. Assassin’s Creed, The Saboteur, and Red Faction: Guerilla are all high-profile open worlds filled with people that do nothing but wander aimlessly. They feel like artificial obstacles in our path. Rockstar is great at creating emergent moments of NPC interaction, moments that occur regardless of our presence. From the spontaneous gang wars in GTA to another gang dragging some poor sap through a town in Red Dead Redemption, Rockstar uses these NPC interactions to make their worlds feel persistent.
I look for opportunities in games to forget my responsibilities to missions and NPCs. Accelerating along the San Fierro Highway with the radio blaring was one of the most memorable experiences for me in San Andreas. Wandering the Capital Wasteland was often more engaging than searching for Liam Neeson. Riding the rails in Empire City was a consistent exhilaration. For some reason, knowing I was the lone non-NPC in the gameworld was a comforting freedom, too.