character traits

Dear Mrs. Naylor my name is Lincoln and I have a question for you. How did you decide the traits for your characters in the story

First I think about the story–the plot.  What kind of boy would put so much thought and energy into all that Marty went through?   Then it’s a matter of imagining the story–what would he be like?  How would he talk?  How would he react?  It’s not as though I make a list of traits and then assign them to different characters in my story.