"What is a character? A factor whose virtues have not yet been discovered." Interesting quote with a lot of meaning in a few words.
When I or any other writer sets out to start a story, much may be known about this character. Reams of material have been jotted down, a character sheet may have been written, characteristics taken from life, psychology, or even Astrology. But the character is still unknown.
Until the character acts and interacts with other characters, with the environment, with his inner fears, the character is unknown. Those characteristics, those lists written down may no longer be what the character is. These traits may have changed as the character changes during the writing of the story.
So to know the character he must be presented with a problem and shown acting and reacting to the character. I never know my characters until I've gone through a story in a rough draft and perhaps even a second re-writing of the story. Suddenly the character develops into a person I know. He is no longer a factor. I have discovered his virtues. He becomes the same and yet different that the man or woman who first leaped onto the page.