There are primary characters, the protagonist and antagonist. Secondary characters are their friends, family, and colleagues. That is about the extent of most stage scripts. But in fiction (and screen plays) depth is added with background characters who are essentially 'extras' to the cast. Without these extras, there would be two people eating in an empty restaurant, walking empty streets, no taxi cabs in New York City...you get the idea.
Sometimes they are anonymous extras like in this airport scene from High Tide (the passenger with the bag).
I walk back down the concourse in a daze. People with bulging carry-on luggage come from another gate and overtake me. One woman mutters, “Excuse me,” when she bumps me hard with her bag as she tries to squeeze between another person and me.
Sometimes an extra is implied. When a phone rings in the background (for whatever reason - maybe part of noise so your character can't hear something said) the phone call doesn't involve the active character in the scene, it is part of dressing the set. It is implied there is someone on the other end of the line making the call. This is unimportant and the reader will easily make that assumption and move on with the action.
Other times, an extra needs more prominence because of a bit part he or she will play later. They get introduced in a way that the reader will recall them later when they return. The gardener in Storm Surge is an example of this type of extra. See his introduction below.
In the distance, a dark figure is slowly moving across the landscape. The figure has tall, spindly appendages that angle toward its back. As it advances, I see a small person with a rake and hoe over his shoulder. Methodically, the figure moves closer.
We rendezvous in the rose garden.
"Good morning, Miss," the figure says and half-bows quickly three times.
"Good morning, I am Kathryn," I answer and offer a handshake.
The little Chinese man removes his gardening gloves and shuffles the weight of his hoe and rake, bringing them down to lean against his body.
Extending his hand, "Juan, I am Juan." He smiles.
"Huang. Mr. Huang, it is nice to meet you." We shake hands. "The gardens are beautiful." I gesture at the rose garden, figuring he is responsible for the beauty.
"I am Juan, no Mister, just Juan."
"Yes, Huang like the Yellow River – Huang He. Right?"
"No, no. Juan Garcia Li." He smiles, revealing a missing tooth from his upper jaw.
"Juan Garcia?" I ask trying to force my mind to make the connection with the Mexican name and the little Chinese man.
He grins again. "My mother gave me American name."
The connection is not as clear to me as it seems to be to him. I nod in agreement, returning his smile.
He smiles again, then starts raking around the base of a rose bush.
Note, each speaker should be indented at the beginning of a paragraph. I haven't found how to do that on blogger yet.
So, if you're interested in writing, there is a bit more about character development. If you're interested in my writing, there is a peek at Storm Surge.