For this book, I will define action as intelligent activity that involves changes of behavior at a fast speed. Examples of action are all locomotion behaviors (a character that runs in Mario), simple aggression or defense (enemies shooting or ducking in Quake), and so on. Notice how action is put in a contraposition to tactical reasoning, which is in turn described as the analysis process used to create a plan that will then guide the actions of an intelligent character. So, action deals with immediate activity, and tactics plan that activity.
Action is thus driven by relatively simple tests. We will need to compute distances to targets, angular separations, and so on. Action is also quite fast-paced. Action games have higher rhythms than tactic/strategic games.