It is critical that you know your character.
No one should read a line in your book and mistake it for any other book.
Find a metaphor that is unique to your character.
Let your character's emotions feel sharply defined to your character.
Every character is in a different world--one in her own body. Your job as an author is to find a way into that world.
Carol always makes her students interview their characters.
How are you going to get to know them if you don't get to know them?
There are a lot of ways to go about interviewing your character. It's important to keep in mind that you don't have to go about it the way Barbara Walters does.
The point in interviewing your character is to get her to tell you her story. Probably you shouldn't interrogate her...
Start with the basics.
What's your favorite color and WHY?
Who do you hate? What happened to make you hate them?
What's it like in the town/city/place you live?
How did you meet your best friend?
The point is to ask questions that aren't one line answers (unless that's how your character is then maybe that's all you'll get). You want to draw their story out of them.
Keep in mind that you may not use all the information from your interview--but the point is to get to know your character, get in her head, understand what her past is and how that has made her who she is or how it has shaped her personality or the way she speaks or how she treats other people. Then write your story once you know who she is.