A physical description helps us to see how they move through the world, and allows the reader to believe in them. Even their name provides a lot of information, indicating age, background and social class.
Who else is in your characters life? Who do they interact with? How do they interact? Personality is often determined by background. What sort of parents they had can play a big part in how they shape up in life.
I'm always asking myself what my character wants from the situation, or the scene they're in, its important to know how your characters will behave. Creating a well rounded character takes time. Not only what they look like, and where they live, but how they will react to the situations they find themselves in.
How do you create your characters? Do you start with a picture? Or an object, and think who might have owned it, worn it, played with it? Perhaps you make a character study? Or do you develop them as you go along, without doing any planning in advance? How do you create an intriguing and credible character?