The latest Resistance 3 Video Developer Diary focuses on two important components for a co-op game: enemy variety and enemy behavior. Sure, it's fun to adventure or run-and-gun with a friend, but it becomes so much more intense and immersive when you can encounter a wide variety of enemies, or when the enemies play smart (or cheap, depending on your mood at the time).
The video explains the "job board" system for the AI where it can assign certain numbers of defenders, aggressors, flankers, etc. so it doesn't just feel like a big mass of enemies wandering around. Enemy types include Grims (who melee or throw debris), Hybrids (who will be wearing less armor this time around), Longlegs (the jumping chimera) and Brawlers (buff, gorilla-type chimera who need to be shot in their weak points to be taken down).
Resistance 3 is due out September 6 on the PS3.