I saw the interview too, but I didn't understand this part. Does it mean that you can solo the whole campaign without an AI/human-controlled partner following you whole the time, or it means you can change other player to control your partner?In Multiplayer, Mercenaries mode will return and will be unlocked from the get-go, and co-op will become "drop-in/drop-out", removing restrictions and making it simpler to play in co-op.
I hope they remove the ORC style 'next door' indicator (at least that's what it looks like to me) or make it optional (on/off), because it looks stupid and makes the game easier (in my opinion):
And they do something with the interface too, while they are at it:
Aside from that, the game looks absolutely GREAT, and finally the zombies are back too!