Then comes the hard part: character – figuring out how this person respond to situations, how he feels, what his background is and what might motivate him to do what he does.
There isn’t room here to talk about all the aspects of personality – may I suggest you study up a bit on psychology, perhaps visit your library and read a few copies of something like Psychology Today? Read the newspaper to see what the interesting folk in your are ae saying and doing and what seems to motivate them. Visit Wikipedia's entry on 16 Personality Factors and branch out from there.
With those suggestions on the side-burner, we can hit some of the highlights and hopefully get you thinking in the right direction.
Is your character more of a follower or a leader? Does he (we’ll continue to call the main character ‘he’ here for simplicity’s sake) act or react? Forceful and assertive or more reserved?
And that leads to shy or bold? Does your character have to be drawn out, or is he the sort to thrust out his hand and introduce himself? Is he one to observe more or participate? Sip a drink alone or socialize?
Think about whether your character is a perfectionist, can tolerate disorder in his life, or initiates it! Does the character realize where he stands in this regard and have some kind of internal dialog about how it affects him? If his apartment is littered with clothes, magazines strewn about and old take-out containers gooey with former contents, does it bother him or is it just part of his life?
How about tension? Is your character relaxed or stressed? Is he hyper to the point of driving away girlfriends and social friends? Or is he so laid back he gets himself fired from a job he likes?
Does he like change and spontaneity or does he avoid last minute dates?
Is he self-reliant to a fault – unwilling to consult or partner with others, but rather doing things ‘lone-wolf’ and getting himself into many sticky situations?
As writers we need to recognize it’s the small things that make the character breathe with life. What is hisr favorite drink? Does he dress predominantly in one color? Is your character afraid of snakes ala “Indiana Jones”? Does he love high speed or skydiving? Does he enjoy crowds or solitude?
It helps to reflect on your own thoughts and personality, but don't make that the only template for developoing your characters.
Having a hard time coming up with character traits from the ground up? You might try thinking about close friends and casual acquaintances - what is it about their personalities that makes one a close friend, the other not so much? How do they match or complement you? How are they opposite you? It can help to sit down and do a short writing excercise in which you describe the inner workings you perceive of that person as well as what shows on the outside. We're all human, we all hide a lot and even in that hiding, expose ourselves.
The next time you sit down to write a story, flesh out your characters in your mind at the beginning and you’ll find that story opening up in ways you wouldn’t have thought of before.