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Spend time world building. Even if you're writing fanfiction, you need to know the world in which your characters reside. I’ve beaten this to death, but that will require research of the source material or history books.
Is it a monarchy or is there a president? Who delivers justice? What are the rights of the people? How is education?
Keep notes and make sure nothing conflicts. It’s just as important to have a seamless world as a flawless plot. You can’t have a world full of contradictions (unless you’re writing Alice in Wonderland) and holes without hurting yourself in the long run.
Plot out your worlds like you would your characters.