I certainly hope that my characters are as strong as those of one of my favorite authors, Sir Arthur Conan Doyle. What I do know so far about the main character for my proposed cozy mystery series is that she will be in her 50's, well educated, with a partner whose job takes him away from home a lot. (I am not sure whether they are going to be living together or not ... more than likely not, at least in the beginning of the series.) She will have an interest in the metaphysical world, and will have studied it extensively. She will live in a large city (more than likely Las Vegas), and will have several close women friends that become involved in some way in her escapades.
A bible is in order, so that she and my other characters grow in the series in a comprehensive manner. IOW, elements of their past have created their present, and their thoughts/actions/reactions in the present will build their future. What else can I do to build my characters into realistic, believable entities?
Charlotte Dillon offers a free, downloadable character chart on her site - CharlotteDillon.com. This is an incredible chart that is comprehensive, makes a great deal of sense, and is highly usable. Note: Please give Ms Dillon credit if you are using this chart in a public manner. This is very much like what I did to create the bible for my characters when I made my first attempt at writing a mystery. Her's is quite a bit more extensive, and I intend to use it! Thank you, Charlotte Dillon!
Why am I not surprised - there is also a wikihow on this! How To Create A Fictional Character. There is a ten step process to follow here that makes absolute sense. As with all things, tweak it until it works for you. It provides a good foundation for building your characters, so you aren't spending a lot of time dead int he water about who they are, what motivates them, and how they fir into the story.
Here is another excellent site, from author/editor Lillie Amman - Creating Fictional Characters.
Last, but not least, from fictionfactory.com -Creating Characters.