When fleshing out your characters, have each one write a short autobiography in their own voice about their lives prior to the beginning of the story, touching on key turning points, memories of special events – both cheerful and tragic, and of the people who meant/mean the most to them.
What are their biggest disappointments, narrowest escapes, greatest triumphs, deepest regrets?
Have them tell you, the author, about their hobbies, religious and political views, hopes, and dreams.
And finally, have your character each write about how the people and events in your story appear to and affect them, from their unique point of view and position in your story.
In this way, your character development will become more organic and more informed.
Keep in mind, however, that our sense of our own selves is usually quite different from how others see us. So, it is a good idea to make sure each character writes about the others in terms of whether they trust them, what they think is their greatest strength, most detrimental weakness, and must fun/frustrating trait. This will not only illuminate how each character sees the others, but how the others see them.
And finally, when you actually sit down to write your characters, don’t just approach them from your author’s view as to what you need/want them to do in the plot, but stand in their shoes and try to understand why they do these things. Is it easy or hard for them to do what they do, compatible or contrary to their nature, and what personal costs or benefits will their actions bring them?
In the end, never forget that we are each the main character or our own personal story, even when we are playing a subordinate role in a larger story involving many others.
Read more writing tips at Storymind.com, the Creative Writing Tips web site.