I think you're pretty right about that, azarhiro. I consider it more collaborative story telling than role playing. In advanced, I often write for a number of secondary and NPC-type characters and I don't feel like I'm just 'playing' my main character. Though as for not having an effect, that's not true, it's just that there are going to be a lot more detail into what's going on and often, in my experience, I've teamed up with the other writer and discussed things before hand so that we can collaborate on a fight or something drastic.