backstory and minor characters
If this term is unfamiliar to you, it just means the sum total of the character's experience prior to the time to story begins. A thirty three year old woman with a degree in biology, a failed marriage, a dying mother and a winning lottery ticket has a tremendous amount of complicated backstory that makes up her personality, her point of view, and the way she will react to the situations you put her in. You created her, so you have to know all about her; you know far more about her than you will ever share with your readers. That's the key: you know, but you don't tell unless it's crucial to moving this particular story along.
For example, maybe this character of yours with the winning lottery ticket (let's call her Naomi) is going to have a rough go of it because she has always been someone who likes routine and order and dislikes surprises. You could just tell your readers that, but you're writing a story and not a case history. You could let your readers see that Naomi always eats the same thing for breakfast: cornflakes, two percent milk, a teaspoon of sugar, a cup of mint tea, a banana. Every day. Day in, day out. You could show them how she organizes her books and cds by title, author, genre, color, size. You could have her interacting with a very troubled, very crazy extended family.
All these things could work, and you might use them. It's easier to pick and chose with a main character, because everything is potentially important.
But what about Naomi's massage therapist? She goes to see this guy once a week every week, same time. It's her one luxury. She talks to him the way some people talk to a therapist, and he provides feedback. His name is Jorge, he's very good at his job, he's a nice guy, he's funny. How much of his backstory do we need? Do we need to know he had a twin sister who died young, and who he misses a great deal? How about his home life, his relationship with his landlord, his politics? If you're writing this story you need to know these things, but you have to be selective about which details you supply for your reader.
Part of the issue is that in reading a story, the reader takes an active part in making it come alive. You may describe your main characters, but the picture you've got in your head of Naomi is going to be very different from the one your readers put together. This is why people are usually disappointed when films are made into movies, because it's impossible to meet the expectations of all the readers.
This is a big topic and not one to be dissected quickly, but if it's of interest to those of you who write, I'll pick at it a little more.