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First and foremost, every character requires a backstory in your mind. You need to know who they are, why they act in a certain way, how they would respond to new situations, etc. Once you have th...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/47131 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47131 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
First and foremost, **every character requires a backstory in your mind. _You_ need to know who they are** , why they act in a certain way, how they would respond to new situations, etc. **Once _you_ have that backstory, you can decide how much of it will be revealed to the reader,** at what point of the story, and whether directly or indirectly. Bits and pieces of a character's backstory can be strewn throughout your story, coming up when they're relevant. It might be that some element would be revealed when this would create the most dramatic effect. Or it might be that the backstory remains entirely in the subtext, never getting officially "revealed". Look for example at Hemingway's _The Sun Also Rises_: the novel starts with the backstory of one character, Robert Cohn. The backstory of another, Brett, is revealed in bits and pieces - here we learn that she's getting married, there - that she's getting divorced first, much much later - why she's getting divorced. As for the MC, we learn very early on that he suffers from impotency, or something similar, due to an injury suffered during WW1. We never learn any more. His backstory is all subtext and no text. Or, look at _The Lord of the Rings_. Boromir tells us his backstory right upon arrival: who he is, where he came from, why, his family, his position in society - everything. Aragorn, on the other hand - it takes several chapters between his appearance as "Strider the Ranger" and the revelation that he is Isildur's heir. Of Legolas's and Gimli's backstory we know little save where they came from, since that's irrelevant; and Gandalf's backstory is a deliberate mystery. But Gandalf's story does exist, it just never gets told. You find out more about him in the _Unfinished Tales_.