How does co-op work?
Generally depending on the scenario, the survivors have some objective(s) that they have to achieve. What they do trying to achieve those goals is often what gets more zombies on the board. Shooting guns, driving a car or leaving it running, walking into buildings are all things that have a chance of bringing more zombies on the board (determined by die rolls). If the survivors are not already in view of the zombies then you move them the location of the original sound, but eventually the zombies will see the survivors and head towards their next dinner. Also, if you happen to run into other survivors who decide not to be friendly their actions are also determined by the game mechanics. "In sight" tests determine if they'll shoot at you when one of your survivors comes into view of these non-friendly survivors. During the gunfight someone will eventually be forced to seek cover or be put out of action. Usually the players can determine the most logical actions of the NPC's anyway. "Your guy is closest and he's in line of sight so I think he'd shoot at you."..."Yeah, I agree."
Campaign
I haven't played the campaign yet as I'm still painting zombies. The campaign covers a rather long time period (20 years maybe?...I can't remember) and during the various stages of the epidemic you are more likely to encounter zombies or other survivors. The scenarios can be rolled up to determine what happens to your survivors, or you can pick one you like in the interest of making the story go the way you want it to. A long the way you might meet other survivors and convince them to join your group. During the campaign you can gain points to improve the members of your group. Sorry I can't elaborate more because I've only read the rules and haven't actually played a campaign yet.
On a side note, I also play Six Gun Sound: Blaze of Glory, another game by Two Hour Wargames and I'm currently playing a co-op campaign with my cousin. It's pretty fun so far (just make sure you have some cover to hide behind!).
Rodney