Remember who you are and who your character is

Most of us probably started playing a character that was rather similar to ourselves. No matter how it looked on paper, it was still played like an ordinary person. If you want to take your character to the next level you need to separate it from yourself!
Always remember who you are and who your character is. It is important to always keep in mind that the two of you are not the same. Make sure that when you write for your character you keep that in mind so that you do not project your own opinions and actions on to the character. Let it live a life of its own!

Make it clear it is your characters opinions and actions you are writing about. Your personal values, even if they are important and you probably want to keep your character within what you feel comfortable with, are not the values of the character. Just like your background has formed you into who you are today, the fictional background of the character has formed it into what it is today. Most likely is it you who has written most of that background, so now you have to live up to what you have written.

A character with a very dark and disturbing background does not react the same way as a character that grew up in a loving family at a protected location.

Think about whom you are portraying and write the characters actions and reactions based on that. You will find that your character will be a lot more believable and perhaps even easier to write for.