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You need the broad strokes of the world filled out before you can really start to run characters through their paces. You can know what your characters are going to do long before you have a finish...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/38790 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
You need the broad strokes of the world filled out before you can really start to run characters through their paces. You can know what your characters are going to do long before you have a finished world but in order to have characters interact with the world as well as each other you need to know _something_ about that world. I usually worldbuild first, probably more than I strictly have to because I used to build RPG worlds and players will always poke at the blank spaces on the map. The world can inform a lot about characters, or not, depending on your exact setting, genre considerations etc... but until you rough in the general outlines you just don't know how much you really need to tell your tale. I'd advise starting with a bit of the world at least, even if only the very broadest stokes.