Our characters should be like this too. There is in all of us that tendency to want to put on a front, especially if we are trying to impress someone. So who are your characters trying to impress and why? What aspects of themselves are they keen to show? Conversely, what aspects are they keen to keep well hidden?!
Which of your other characters can see right through this behaviour? (Do they do it themselves or, say, expose Character A's weak points because they want to impress Character Whoever and don't want Character A as a rival? Obviously really relevant for love stories).
Do you sympathise with your characters' weaknesses? Why do they have these particular ones? How do these affect your stories?