With the overall game design for your game in place, you’ll already have a rough idea of what the characters should do in the game. AI design is the process of refining and decomposing this idea into more specific behaviors.
When designing your character behaviors, be sure to focus on the player. This will make your AI seem much more intelligent and responsive from the player’s perspective, and it will reduce your workload too!
Start by working out the major situations that each character can be in. For example, for a stealth/action game, the enemies would have the following behaviors depending on the location of the player:
Idle — No contact with the player.
Suspicious — The player is presumed to be nearby.
Attacking — The player is visible, and targeted.
Your first version of these behaviors should be very basic; don’t worry about details at first. Then, in turn, you can break down each of these behaviors into more specific ones.