believable characters

In order to create a believable story, you have to create believable characters.
The only way to create believable characters is to know everything about them: physical aspect, behavior, psychology, sociology, ideas, passions, their life before the beginning of the novel.
It doesn’t matter if in our novel will be shown even one day of the life of our characters: the way they will live that day, every single gesture, will be the result of their way of being and experiences lived until the day before.