Every AI is different and how that AI runs and thinks, is determined by the effort put into it. The more effort put into an AI, the smarter it will be. Before you get to coding your AI, there are a few things you need to think about. How do you want your AI to act and how do you want him to react. A reactive AI allows the player to feel immersed in the game.
Say you shoot at an enemy. Will the enemy stand there and fire back, or will he take cover and react to the fact he is being shot at. Situation like this are what get the gamer hooked to the game, but also requires more effort by the programmer. The programmer would have to code objects that the AI detected and read as a cover object. You would then have to program the AI to move to cover and get behind it to avoid harm.
Below you will find ways to code your AI, Concepts to consider for the AI, and ways to make an overall fun experience with a computer intelligence.