Coming up with believable characters.
First off make them real, and real people are flawed. Everybody has a flaw of some sort. It may be physical in nature, a personality trait, a bad attitude, they don't speak well, evidence of some past injury, they dress badly or oddly. It can be any number of millions of characteristics that describe real people. So make your character real. Give them some flaws through your writing.
A perfect place for inspiration is at your local shopping mall, or Walmart store. Just open your eyes and remember what you see and notice about people and then give these attributes to your characters. One of the ladies in my book club gave me a sweat shirt that says "Be Careful I may put you in my next book." I use things from my own life, people I know, and am constantly, attentively watching people I run across in my everyday life for attributes to use with characters in my stories.