Just like the characters in a stage play, it's equally important for the author to portray their characters with consistency, depth and conviction.
When I start to write a story, I find that the story is lacking and the characters are shallow until I get to the point in time where I begin to truly understand not only the characters, but also their motivations. Part of the character development is in the builiding of the hidden elements of the character that may never be explicitly described within the story. For example, it may be an integral part of the character's development, but not necessarily as big of a part of the story and plot itself, that the character had been adopted.
I was recently reading a book of quotations and I came to the realization that specific quotations can be highly indicative of the character's personality and values. As such, when I read a quotation now, I ask myself if that quotation might have personal meaning to one of the characters in my story and this in itself can be a building block of character development.
In my work-in-progress-debut novel "Pieces of Silver", my protagonist regularily posts quotes that are meaningful to her on her bulletin board as a way to "show, not tell" the state of her character, her values and her philophies.