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What I do as I'm discovering my characters is to just write them into scene after scene. Don't worry about whether the scene has anything to do with your story. Put him in an argument with his wife...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/20124 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
What I do as I'm discovering my characters is to just write them into scene after scene. Don't worry about whether the scene has anything to do with your story. Put him in an argument with his wife. Sit him next to a drunk at the bar. Put a pebble in his shoe. Have the neighbor knock on his door at dinnertime. Just put him in a bunch of scenarios and write them. He'll reveal himself to you. Then, because this is just discovery, we pitch all those scenes and get to work on the real story with a new understanding of the character. (This discovery phase could even change some foundational elements of your story.)