From the looks of things the game's AI has been beefed up significantly since journalists previewed the game to mixed reactions last fall. There's a whole new aggro system in place which allows one player to draw the enemy fire more than another. The advantage of this is it allows the secondary player to sneak around and take out badguys from the back. Nice! Being a co-op game, what would happen if you'd try to leave your partner behind? Well the WSJ asked the same question.