I've been blogging on how to write in a more meaningful and literary fashion. One critical character type that you'll need to include for almost every kind of story, whether fictional or factual, is the minor character. Minor characters flesh out your world, make it feel more realistic. Without them, it's just your main character(s) and their antagonist(s). That can work for some books and short stories but not many. Most stories need inhabitants that people your world, and you'll need to give some thought to how to craft those characters.
One kind of minor character is someone who may be a name and/or face who is part of the crowd, one who doesn't play much of a part in your story but simply serves to texture the world. These characters can be given some slight description, a name, a face, but they should generally be unmemorable. They mostly serve as a backdrop. They may interact with the main character but only in minor ways. These are the hotel clerk, the main character's student and/or her parent, a witness to a crime, etc. Think of Rosencrantz and Guildenstern in "Hamlet." They started out as his friends, became his enemies, and eventually ended as pawns in his war against his father, used by both side.
If they serve any part beyond just peopling your story, it should be to highlight some aspect of your main character, to make them stand out. Rosencrantz and Guildenstern highlight Hamlet's progress from bystander to blade of justice in his late father's ghostly hand. If your protagonist is running for his/her life, the minor character is the distant sibling ignorant. of the hero's danger or the tourist simply enjoying the sights. They're living the peaceful life your protagonist wishes to have. They're bodyguards of the enemy who show the competence or lack thereof of the protagonist. They may stand out briefly or steal a single scene, but they shouldn't overshadow the main character. Every character that stands out in any way should be there to advance the protagonist and/or the plot, in whatever small way. One of these characters could even start out part of the background but turn out to be a secondary character or an antagonist or an important key to the mystery, a rifle on the wall.
Another kind of non-protagonist character is one that plays a significant role, a secondary character. This is the kind of character that can be memorable, can steal scenes, may land clever lines and star in pieces of the story but should not be the main heart of the story. Subplots can revolve around them, but the main plot should engage directly with the main character. If the main goal of the antagonist is to thwart a secondary character with no connection to the main character, your story will feel disjointed, like you chose the wrong protagonist. Ron and Hermione can take the lead for certain parts of the book, but Harry Potter is clearly the perspectival character, the protagonist. In the aforementioned "Hamlet," Horatio is the only character who comes close to knowing the truth behind Hamlet's charade.
These secondary characters help the protagonist grow, give them the opportunity to interact in order to illustrate aspects of the character, and provide side conflicts and comic relief. They mirror and contrast with the protagonist's choices. These are friends, significant others, and other associates that mirror and expand the plotline. Their critical role in the story is to help the main character grow and pursue their main goal. They challenge the main character and help them grow. They enlighten aspects of the main character even the main character doesn't understand. They may be threatened to help the protagonist rise to greater heights or even sacrifice themselves on some level to help the main character succeed.
How are you using your side characters? Do they ever take over as the protagonist or overshadow your main character? Do they always serve an important enough role to justify their existence? Why is each one there? How do they improve and enhance your hero's voyage?
