In Half Life, when they yell "cover me", one tries to flush you out and another cuts you off. What's really interesting is what all the other soldiers do: they run to random positions. Early builds didn't do that and the AI was just plain mean. Game AI is a constant balancing act between being good/engaging and not killing the player.

Civ is the same: the AI has routines that try to win, but also a whole bunch that role play.