“The measure of a man’s real character is what he would do if he knew he would never be found out.”
You identify yourself by what you do when no one is watching. The supervised athlete may be the hardest worker, the most selfless and responsible competitor. But how he practices when no one sees him, how he interacts with teammates when the coach is not within listening distance -- that's when he defines himself. His character.
Theodore Roosevelt extended the definition to self-awareness and independent self-evaluation saying, “I care not of what others think I do, but I care very much about what I think of what I do. That is character.”
From “Coaching The Mental Game” by H. A. Dorfman