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There are many ways to develop characters for your stories, but I've found the best way is with an extensive character assessment. In the interest of brevity, I won't be posting the entire assessment I use, but will encourage you to seek out an assessment of your own that includes some of the following:
Typical expressions when happy/sad/angry/frustrated/etc.
Parents: What was important to the people who raised him/her?
Strongest/weakest character traits
How is his/her situation ordinary or extrodinary?
How do people view this character?
Overall outlook on life
What personal demons haunt him?
Does he/she have a secret?
As you can see, an extensive character assessment goes deep into the person. Whether you use it all in your story is an individual decision. You will, however, be able to create a more three dimensional character with this information and you will know your character and how he thinks when you are writing him into situations.