Previously I have touched on how to find your characters, how to describe them and how to give them personality. I am going to expand on this a little more.
You have to be aware of every facet of your character's personality, because if you don't know it all, you won't believe in the character enough to present them as a rounded, credible person.
You can't go into a full physical description and personality evaluation of each character as they appear as this would be most tedious. Instead, you need to work out what the readers need to know and when they need to know it. Then you can introduce other information during the story without making it obvious. The correct method of describing characters is best summed up by the phrase, "Show, don't Tell." Here is an example:
Example of Telling the Reader
Mary was a voluptuous red-head, but she wore too much make-up for a seventeen-year old.
Example of Showing the Reader
Mary added another layer of lipstick and powder, preening in front of the mirror and thinking "I knew this shade would go well with my hair. I hope Mum doesn't realise I pinched her 'Coppernob' rinse. Perhaps I should take another tuck in the waist here, just to nip me in a bit more."
I hope that you agree that the second example reveals far more about the character and delivers the detail in a subtle yet interesting way.
I hope that you find this and my other tips useful. Good luck with your writing!