I was thinking that it could be mostly serious, but have the occasional funny moments. Not exactly the most defined answer, but it's the best I can think of.
As for players, I don't think we need more than three, really. We should be able to manage NPC-like stuff ourselves. I was also thinking that the extra-mortals should be able to affect people, influence them to their own ends. (with limitations, obviously)
Though I suppose we could perhaps have one other person to run the 'world', as it were. (Not quite sure what the technical term for that would be)
As for who plays who, I'll go ahead and pick the demon, I think. You can fight out the rest yourselves :p.
Umm, I think that's everything. Anyone think of anything else?