I don't think that's how all RPing goes down though. Several of my roleplays we have chosen a certain character type or relationship, developed characters and then created a world to fit them in. The most expansive world I have created with my partner currently started as a pairing we wanted to write, we built the entire world around them and then got deeper into it. As we discussed all the different facets of the world, the characters became more developed too so really it doesn't matter which we started with, by the time I wrote the opening post we had developed both of them together.