1.) Know the answers to the questions (listed 2 paragraphs above) about each of your main /secondary characters.
a.) "Inspiration For Writers" has a GREAT, very detailed Character Traits form you can fill in. It's the perfect place to start!b.) "Personality Page for Writers" allows you to find your character's myers-briggs type and all kinds of other fun ways to peg who your character really is.
d.) Fiction Factor has a moderately helpful article on Characters' Physical Traits: What to include in your story and what to, maybe, leave out.
Now, of course, you may not have time to do all of these "character building" activities. Pick and choose what works for you.
The main thing when writing a character is to know enough about their inner wants and needs, and their outer appearance, actions, and relationships, to make them appear real on the page.
We want our characters to be so real that readers around the world will know exactly who others are talking about when they say "what's that kid's name? You know, not that wizard kid with the scar whose parents were murdered, but his friend, the blonde girl who they all thought was stuck up, and who is almost too smart for her own good?"
Yep -- the whole world knows Hermione... and Ron, Harry, Anne, Pippi, Gollum, Gandalf, Hamlet, Clark Kent, Charlotte the Spider -- and many of us also know silly little Clover sitting on Levin Thump's shoulder, above.
So let's all have fun learning about our own characters and making them similarly unique and engaging!