Because I've been the judge for many writing contests and was a writing instructor for Writers Digest School for many years, I've read many self-published books and manuscripts by new writers. One of the problems I've seen over and over is lifeless characters, or characters who are no more than a name.
Often it's not because the author doesn't know his or her characters, but rather the problem is not knowing how to develop the character on the page so the reader will know the character too.
First, each character should have an appropriate name: a name that fits his or her personality, a name that fits the type of book, the time period, a name that doesn't sound like, rhyme with, or start with the same letter as another character. The author needs to do everything possible to keep from confusing the reader.
To make sure not to give wrong information about someone, the author should have the facts about each character written down so that the hero doesn't suddenly change eye or hair color half way through the book.
The author should know enough about the history of the characters so that the motivation for doing things, or reacting in a certain way rings true.
With dialogue, does each character have a unique manner of speaking?
Instead of always using dialogue tags like he said, she said, using an action by the character who is speaking or a description as a dialogue tag, can be another opportunity for telling more about a character.
Some authors keep lengthy notes about each character which can be very helpful.
I've been writing about my heroine Deputy Tempe Crabtree for quite a few years. I know her better than I know any of my relatives or friends. That may sound strange, but I am totally aware of how she thinks, why she thinks it, and how she'll act in any given situation.
When writing about any point-of-view character, I try to "climb inside" him or her and see the world and what is going on through that person's eyes, hear what they hear, smell what they smell, feel what they feel, both emotionally and by touch. This works for me, perhaps it will work for you.
My next Deputy Tempe Crabtree mystery is Judgment Fire from Mundania Press http://www.mundaniapress.com
To learn more about me and my books, visit http://fictionforyou.com