A common mistake novice writers tend to make is creating characters that are too perfect. Not only is this highly unrealistic, it's a turn off. Imperfection opens the door for more outcomes and conflict; elements that spice up the story as well as your characters. So don't be afraid to give him or her flaws. After all, even Superman had his kryptonite.
A strong character has a defined disposition, issues, aspirations, hobbies and beliefs. Give depth to each character's personality. Most of his or her history may never grace the pages of your novel, yet just like people, a character's background defines much of who they are and why they behave as they do. These details will help prevent two dimensional personalities which are generally caused by lack of back story development for characters.
Include quirks that make your character unique. These attributes will give your character charm and flare that your audience will remember. Whether it is a catchphrase, tick, accent or habit, create a quality that is solely theirs. Personal appearance and style are other distinctive forms of self-expression, so feel free to expand on them in ways that reflect the character and help make him or her stand out.
Raise a conflict that will rouse concern over your character's well-being. Exploit their weaknesses. It sounds cruel, but by challenging a character the reader cares about, if not directly threatening their well-being, you will pique the reader's interest and he will want to keep reading.
Pay attention to how the characters speak. If your character is from England or New York, expand on that. Give them the proper terminology to use. Make sure you research any dialect you use that's unfamiliar to you. Another important detail to consider is how your character participates in dialogue. Conversations consist of more than just words. For example, if your character is a Italian stereotype, referencing hand gestures as he speaks will help the reader visualize him and infer his cultural background.