One of the most vivid aspects of character is the character's appearance. Is the character male or female, old or young, conventionally attractive or not? Be aware of the physical attributions of the character, including not only appearance, but also what the character chooses to wear, how he grooms their hair and what beauty products he uses. When writing your own fiction, avoid merely listing your character's traits; instead, reveal them through the character's interaction with others.
Do your characters speak with their hands? Do they smile often or seldom? Do they look other characters in the eye while speaking or look away? Think about mannerisms and how they can convey a character's personality and ideas about life and the world. For example, what does the way a person stands at a party say about him? How does a person who stands in the corner with her arms crossed differ from a character who goes toward the center of a room and offers hugs or handshakes?
When reading an existing text, notice the small details of how characters use their hands, eye contact or how they fill physical space. As a writer, keep these characteristics in mind when you describe your character in relationship to others.
The way that a character communicates with others should distinguish her from other characters in a book. Think about how a character speaks, how often and at what length. Is your character quiet but effusive when she finally does speak? Does he speak frequently but in terse statements? What phrases does he use? All of these questions should be answered when both describing an existing character and in creating your own fictional character.
A fully formed character should have a history. How does the character feel about her childhood, where did she grow up, does she have siblings, and did she do well or poorly in high school? These questions, though not always front-and-center in a book, should be in the background, informing the way that a character acts in the world. A character should be a like a person in the real world: multidimensional and with a history that shapes his personality.