This is also true of the details used to describe the characters in your novel. It is possible to go on and on for pages without imparting anything of any value, especially when discussing your characters. Think about the last time you asked someone to describe a person you'd never met. You'll get responses like, "He has brown hair," "He's five foot nine," or maybe, "He's got wide shoulders." None of that tells you anything important (and certainly doesn't help you pick him out of a crowd), so you ask what he is like. You'll likely hear the familiar refrain, "He's nice." Or maybe the opposite, "He's annoying." Again, this tells you nothing. Five minutes of your life you'll never get back.
Most people are really awful at describing someone in useful terms. Most descriptions are just too general, and generalizations are completely useless in fiction. Almost everyone could be described as nice in some situations, but annoying in others. These are not individual traits, and the art of characterization (at least in fiction) deals with the individual. So your job as an author is to make sure the reader understands how this character is different from all other people.
To accomplish this, you have to provide details that are specific enough that the reader can begin to construct that character in their own minds. If you do your job correctly, they should almost be able to see the character your describe. Every detail doesn't have to be completely unique to that character (let's face it, more than one person has blond hair, and many people shout when angry), but the combination of details should add up to construct a character who is immediately recognizable.
So how do you do this? First, choose the right details to convey character. Being scattered about it just doesn't work, and irrelevant details bog down your story. Don't describe the birthmark on a character's foot unless it has something to do with your story. Do they use the birthmark to identify the body? It is used in the plot in any way at all? No, then leave it out. A detail with no purpose becomes ridiculous. Make sure the plot is motivating you to reveal that detail.
Now that you've determined what details are important, you have to decide how to reveal them. Don't do it all at once because it will come off as a list. No one lists the specific details of anyone unless they've been asked to (which only rarely happens, by the way). Is a character's hair getting in her eyes? This might be a good time to casually mention her hair and eye color. Does she have pale skin? Or dark skin? Is there a casual way to reveal this (hint: Yes, there is)? You can always have another character notice physical details (as long as your using third person point-of-view), but again, only mention then if they're important to the story in some way.
When it comes to a character's personality, which is the most important part of characterization, the only good way to do it is to demonstrate personality. Use dialogue effectively to show how your character reacts to given situations. And if you've got any internal dialogue at all, make sure it reflects your character's personality. To do all this, you have to know your characters as well as you know yourself, so take some time to get to know them. Then you can impart the pertinent details to your reader.
There is a delicate balance when it comes to the details you'll need to create an effective and interesting character. Choose your details carefully, and remember that a detail is only as good as the reader who recognizes it and understands what it means. So be clear, be concise, and don't be afraid to hit 'delete' and start again.
