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What you need to do is show your introvert to the reader when no other characters are looking, or when he's with people he's close to in some sense. Does your introvert cry when he goes home from...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/6849 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/6849 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
What you need to do is show your introvert to the reader _when no other characters are looking,_ or when he's with people he's close to in some sense. Does your introvert cry when he goes home from his job? Write in his journal or a private blog? Kick himself for not speaking up about something? What about when he talks to his parents, his brother, his landlady? Does he tell him things he wouldn't tell his boss? Another similar technique is **[WARNING: TVTROPES LINK]** called [Pet The Dog](http://tvtropes.org/pmwiki/pmwiki.php/Main/PetTheDog), which shows us an otherwise Jerk character doing something nice, so we realize the Jerk has a heart of gold under the crusty exterior.