We all know what a character is. We see enough movies, we read enough books, we have the concept thrown at us in school through representations from Shakespeare ... and of course our lives are filled with characters. There are small parts like the people who fill your coffee cup or exchange your money at the bank. There are extras like the others driving cars around us, or filling the queues and theatres, or crowding the square at New Year's. And there are those characters who pose as major supporting players in our lives.
A quintessential quality about reality is that at any time, for any reason, one of the extras can spontaneously be thrust into the role of major supporting player. Ten minutes ago, she was sitting next to you on a bus, without knowing anything about her. Now, she has just saved your life by a hair's breath, having her thigh cut open as she pushes you into the aisle just before the bus is hit by a car. She falls into the aisle next to you, crumpled metal and plastic all around you. You're tearing off your expensive suit jacket and pressing it into her thigh to staunch the blood as she starts to shake from shock. You're holding her hand and reassuring her ... and at that point it's time to find out who she is.
Sadly, writers find themselves casting around for some expression of what makes this woman tick, and they don't have it. A rather pathetic description of the woman's hair, size, eyes, age, ethnic background and so on is gorged out like the boxes on a tax form. Some expression of the woman's social connections is attempted and we find out she has a child - as though this is the only possible association this woman could have - or that she has a mother. Two paragraphs of description later we know nothing about her except a few physical descriptions we will forget immediately (because we will see her in our mind how we want to see her anyway) and a thoroughly dismissable family connection.
It is hard to explain what a writer can put there that isn't trite and useless to the reader. I can say that in those moments of stress, you as the passenger trying to save her life won't care about any of that. For you, there is blood everywhere and you are probably inadequate to the task of managing it. You will be hoping that help comes soon. You will be encouraging the woman to talk because you will want reassurance that she is still alive. You won't care what she says - unless it is something so out of the ordinary that you have to listen.
The problem comes from the rather clumsy set of priorities a writer has, in that first the idea of the bus crash is created, and then it comes down to personalities. Let me put it plain: plot does not drive character. Character drives plot. Logically, you should have the purpose of this woman set out in the story ahead of time. The bus crash is the best way you can think of to relay this character's contribution to the story - and therefore, you will have ahead of time the priorities you need to tell you what information the woman must relay before she falls unconscious or dies.
If a bus crash isn't the best way for her to relay it, then you shouldn't write that into your story.
Before setting out to write a story, you want to have two characters sorted out quite clearly in your mind. You should know how they will interact with each other. You should know what agenda both will have, and how those agendas will conflict with one another. The appearance of both characters will descend, therefore, from the agenda, and NOT from a momentary visual you got from sleeping, the television or some other source.
For example, if your first main character is a professor, and your second main character is a student who falls in love with him, the student's appearance will have a lot to do with whether the professor will make commit an act generally seen as amoral. The professor's appearance, and the professor's personality, will need to be something a student can fall in love with. Matters will be different if the characters are both the same sex, and they will be different if the authority figure is a man or a woman. The motivations behind the student's interest in an older authority figure will need to be expressed, as well as the motivations behind the professor's willingness - or lack thereof - to allow the advances of the student. The appearance of either will also depend upon which participant is the more aggressive. An aggressive student who is successful will look different than an aggressive student who is unsuccessful.
I have thus created the pattern and structure of the book, and I have a reasonable idea of what both characters must look like in order to make the book believable and accessible. Now I can start on the plot.
I will be writing more about character first, however ... as there is more to be said.