Most games that contains big cites doesn’t fill the buildings with proper interiors because of the cost in time and performace to do so. Instead they only apply a texture on the buildings exterior and make interiors only for a very few buildings that are of special interest for the player. With this method you can fake the interior of buildings without the need of much extra work from the artist, programmer or designer. The result can give very living cities.

Link to the paper describing the interior mapping in detail:
http://interiormapping.oogst3d.net/