Learning the backstory of your characters can happen several ways. Some writers spend hours creating the past life of the characters, sometimes writing pages of backstory. Will the writer use it? Maybe. Other writers discover their characters backstory as it becomes relevant in the story they are creating. The important thing is to understand when and how to use it.
Prologue, memories, flashbacks and bits and pieces can all convey backstory. Remember the reader only needs enough information to become engaged in your characters.
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