User blog comment:Willbachbakal/The Co-Op System/@comment-175.139.62.248-20130905041231/@comment-75.36.141.136-20130905081902

And (depending on how competant AI running a non-player main character) would vary the play (giving more roles? variied by different abilities??)  for 2 or even just 1 player.

Of course You could also do that with a varying set of companions (like Dr Who) who are just add on NPCs