If you're writing a piece of prose fiction, you want to give a few details that will identify that character
This section of this article is about introducing your main characters (hero, protagonist, antagonist, villain, love interest, etc.)
Example: "The woman in the yellow trench coat.", "Dan always wore a cowboy hat".
Show don't tell. This should be a practice you adopt for your fiction. What that means is; instead of directly telling your reader what your character is doing. Here is an example of telling V.S. showing.
Mary opened the door. She hated Todd and now she had to be nice to him or she couldn't go on the big school trip.
Just as she predicted, there he was sitting in the back row on the left. Her legs felt like bricks as she made her way toward that no good snake.
Giving her best fake smile "Todd, I was wondering if you wouldn't mind -of course- make an exception and let me go to the museum on Friday with my friends?
Tom, grinding his teeth, approached the door. His legs felt like bricks as he turned the knob.
Notice the use of description in the showing example. "Grinding his teeth" lets us know Tom is not happy about something. Other descriptive words adds to this feeling of dread; grinding his teeth, legs felt like bricks, no good snake.
It is important when introducing your character to do the same. Your reader won't feel anything for him/her if they can't share in the experience with that character.
Having a long list of background information about your character even before the story begins is an easy way to lose your readers. You don't want to bog down your readers with a lot of extra information when they meet this character. Its better to give out details about your characters slowly.
Sometimes the information you want to let the readers know about your character is better off shown through an action. If your character is a pick pocket though his family doesn't know this about him, show him picking a ladies pocket at the playground while he is watching his son on the jungle gym.
Show with action.
Try introducing your character through an action. Describing an action can be more telling of a character’s personality then trying to describe who your new character is to the reader. If your character is an evil person, introducing your character as a 'low down scoundrel' isn't as effective as he or she kicking an old man in front of a bus to save him or herself .
Introducing your new character in the middle of a situation will also make that character shine. If you're writing about a Super hero with super strength, introducing him by having him catch the tractor trailer before it smashes into a little kid will leave a lasting impression in your readers mind.
The same rules apply as with your main characters. With minor characters though, you can just have them show up. They come through the door or was in the audience, for example. Minor characters can just be minor.
One rule regarding minor characters I think everyone writing a story should ask him or her self; is the character necessary?
Your readers have enough characters to think about, do they need one more? Here are some questions you should ask yourself before adding that minor character:
Does this character add to the overall plot, environment, or theme of the story?
Will the story be resolved if this character is not included?
Does this character distract my readers from the important main characters and plot?
If the last question is true, maybe that minor character needs his/her own action.