Plotting out a novel is a wonderful way to prepare for writing. It helps an author identify which thoughts, events and scenes are needed and which are not. When significant effort is invested into plot development, tremendous amounts of time can be saved during writing. Scenes which seemed necessary during early contemplation don't have to be written at all, if they've been eliminated during the plotting process. Plotting is therefore a method which optomizes the writing experience; minimizing the total number of words which must eventually be written.
Charcter development is the exact opposite. Characters are organic. They only grow through experience and exercise. For characters living in a literary world, that growth comes in the form of written words (lots of written words). Ten to Twenty "never-to-be-published" pages, containing a freeform written monologue in a character's voice, will give you a real idea of who they are, what they value, and how they use words.
Your plot may help you realize who each of your characters need to be (for the sake of your story), but you will only discover who they are, by letting them rule your writing hand for a while. "Become Them" and let them write. Only by imagining the world as they might see it, can you truely figure out how each of your characters fit into the world that you are creating.
Here is my formula...
I usually extract a list of characters from my plot and sort them by importance. "Main Characters" will get the most development time. "Supporting Characters" will get a little less, and so on.
I then chart out which characters know each other, using colored arrows to summarize their opinion of or affinity to each other. A red arrow from Joe to Bill means that Joe hates Bill. The yellow arrow from Bill to Joe suggests that Bill is unaware of Joe's antipathy and only distrusts Joe but doesn't hate hime. The colors are arbitrary and I often scribble notes along the arrow's length to capture more complex emotions.
I will then write up a short 3rd party summary of each character, using omnipotent view. Sometimes I use a character profile form for this, to capture each character's occupation, wealth, posessions, etc. These are trivial facts but they set some ground rules which will later be followed by the characters, when each of their turns come to write.
Finally, I hand over the metaphoric pen and let one of the characters start to speak. I start a new word document for each character and try to keep the writing in that document to one character's voice, staying strictly in first person point of view. The characters usually start by introducing themselves and then babble on about anything which they choose. Sometimes they talk about other characters. Sometimes they share stories from their past. This can be very surprising and instructive to me, the author. I let them continue until they either run out of things to say, or until something strange happens. After each character leaves the stage, another takes its place.
Strange things happen a lot during these free form dialogs. Other characters may walk out on stage and start discussing the current subject with the speaker. Sometimes there are arguments. Tears are a good sign that things are getting real. Laughter is too, to a lesser extent. Late in the process, pairs of characters sometimes fall into a smooth banter with each other, playing off of the other's word choice and style. Through out these events, I try to stay in the original speaker's POV. In this way, their private thoughts and responses can be captured, along side their public words. Later, writing the same scene from the other character's POV can be very really illuminate what was really happening on that stage.
The idea is to capture who each of these characters are... Their thoughts in their words, spoken with their particular subtlety or style.
The pages generated during these monologues are never included in the final novel directly, but I often go back and re-read them just before bringing a particular character on stage. It is helpful to remind myself of who they are and how they speak. All of it helps me to make them real when they finally get to visit the to-be-published page.