Writing well-developed characters is a true talent. When you write well-developed characters, your canvas of imaginary friends and foes comes to life. People "buy" into the idea of them and emotionally invest in them. They talk about them as if they were a friend or neighbor or associate. The first time someone talked about my heroine Brea Harper in California Girl Chronicles like she was a girlfriend, I was somewhat taken aback, but then I realized this reader had made the necessary emotional investment in this character.
So, how do you create well-rounded, interesting characters that readers will invest in? Here are some tips on how to do develop your characters.
Don't rush anything -- many new writers will not only hurry through their story-telling process, but also try and force their characters to life. What does this mean? They will try and develop the character by telling the reader all about the person in a paragraph through description. When you first introduce a character don't force it. You can briefly describe the person, yes, but don't falsely believe a paragraph or two and you've done the job.
Show it don't say it -- let the characters reveal themselves on the page by showing their behaviors. Put them in their places in the story and then have them behave. Through their behavior you get to know them. For example, in California Girl Chronicles you have the flighty, mercurial Letty from the bikini shop. She is shallow, gum chewing, self-absorbed and colorful. Through her appearance (different colored hair and piercings), we gather she's rebellious and then she acts rebellious and shallow at every turn.
Consistency -- then make sure you keep your characters consistent. If Letty is shallow in one scene, she is not going to miraculously change her behavior to deep and caring in the next scene. Now if you intend to make her somewhat crazy and erratic then use this tool, but keep it in context and allow the characters around her to notice she is nuts. If you've been inconsistent with a character out of mistake or not realizing it then it's an error and not planned.
Dialogue -- what characters say matters as much as what they do. Use the dialogue to develop their attitudes and backgrounds. Remember, most people don't talk in soliloquies and speeches. Use pedestrian language and keep it real. If a character is educated, use dialogue to show they have a vocabulary. If they're urban, use the dialogue to show that background. You can also easily define characters by words they don't use, too. If a character isn't profane then you should avoid profanity being used by that particular character. If they have a dialect or accent, make sure you "imply" it and never misspell words to sound out the accent.