prosfilaes
Adventurer
Sure they're playing roles. Most likely, they're playing multiple roles from a stance similar to how DM's play NPC's. Or, they could be focusing on a single role, much like a DMPC.
Again, aren't you arguing that DM's can't roleplay?
A DMPC is when the DM is trying to be a (non-DM) player and a DM. If you have a DMPC, then you're taking a player role in the game, even if at the same time you're taking a DM role in the game.
I don't believe it's a roleplaying game without a player, without someone firmly adopting a role. Even if you're roleplaying multiple characters, there's still something different about being character-focused, and driven to interact with a world via a character or characters, or being god-mode and interacting with a world using characters to tell a story.
I'd argue that no, collaborative novel writing isn't generally roleplaying, but, probably because collaborative novel writing isn't generally done sitting around a table together.
Neither are a lot of RPGs, any more.