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You can do both. You can start from an existing city (maybe give it a new name) and change the areas which do not fit (GTA comes into mind). If you have already an idea for the setting like "Oh, t...
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You can do both. You can start from an existing city (maybe give it a new name) and change the areas which do not fit ([GTA](http://en.wikipedia.org/wiki/Grand_Theft_Auto_%28series%29) comes into mind). If you have already an idea for the setting like "Oh, that should play in New Orleans" because of the (cliched) reputation of that city, then go with that. Your New Orleans does not need to be square inch by square inch like the real one. You want to set atmosphere in your book, not a geographical map. Generally if you can't decide, make plans for both options, what you have to do to accomplish the goal. Then pick the more frightening. The bigger the road block in your mind, the more your subconscious wants to get over it.