It's been said that character is defined by what you do when you think no one is watching. What an illuminating concept that is.
Most of us have a public face and a private face. There are parts of ourselves we don't want the world to see. Typically, we tend to hide the aspects that would not be viewed favorably by society. Greed, lust, jealousy, pettiness, fear and so on. We also tend to hide our weaknesses. No one needs to know that we can wolf down a gallon of ice cream...