Because conflict is the key element in any form of story telling, it is important to make sure your characters are different in significant and minor ways. Characters with differing world views, attitudes, values and modes of operation offer a wealth of opportunities for conflict.
The by-the-book cop partnered with someone who thinks the rules are merely suggestions meant to be broken.
A neat freak living with a slob.
A night owl married to an early riser.
A penny pincher and a compulsive gambler.
A daredevil into extreme sports and a fearful homebody.
These are just a few examples of character couplings with built-in conflict possibilities.
Not only will these characters have a natural conflict simply by being in each other's presence, they will disagree about how they go about solving problems and achieving the major goal of the story.
Take a cop story, for instance. Say one character wants to bust the drug ring by carefully building a case, while the other want's to bust in with guns blazing. Great conflict.
Having two or more characters who want to achieve the same goal together, or for an even greater level of conflict, two wildly different characters who are, for whatever reason, forced to solve the problem together, can create several levels of conflict.
So when starting your next story, create characters who will naturally conflict with each other. It can lead to great story telling.
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