Building a character and building a person are, relatively speaking, the same thing. Just go ahead and start designing the character, and then, at intervals throughout their creation, ask yourself, “Is this what a real person under these circumstances would do? Is this how they’d act? Would Iact this way in the same situation? Would anyone I know act this way? Is this the kind of character who would even have a ‘human’ reaction to these events?”
Doing the above gives us a sense of reality in relation to the character, regardless of whether they’re a man, a machine, an alien, or whatever. When we can observe the character realistically, no matter how strange they might be, they’ve been designed properly.
AN IMPORTANT NOTE: designing a character purely for interest is not interesting. We’re not interested characters because they’re interesting. We’re interested in characters because they themselves are interested in the world around them, thereby becoming “interesting.” We are interested in the interested.
Realistically Designed Characters: Han Solo, Andrew Detmer, Commissioner Gordon (BEFORE The Dark Knight Rises), Andrew “Ender” Wiggin, President Josiah Edward “Jed” Bartlet.
Characters Designed for Interest Alone: Jar Jar Binks, Mudflap and Skidds (from Transformers: Revenge of the Fallen), the Catalyst, Gurgi (until the later books that is).
In short, make people, not characters. And don’t force them to be interesting, because that’s not interesting.
~D.
Dylan Alexander 
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