Characters are depicted through both narrative and dialogue in a work of fiction. They can be flat or minor, or round and major, developed with more depth. The persona is revealed through the character's responses to conflict, through dialogue, and through descriptions.
Characters in fiction can have many roles and purposes, all of them dictated by the writer's intent and style, working together to intricately move the plot forward.
The protagonist is the main character, the hero or heroine of the story. In some cases, the reader experiences the story through this character's eyes. In others, the protagonist might be only one of several characters whose perspective is described.
The protagonist does not have to be a character with whom the reader identifies. He might even be an anti-hero, unpleasant or even evil, and that's fine if it furthers the plot.
Or she might be a true hero but could also be a character the reader is supposed to dislike due to a certain character flaw or circumstance. Think Becky Sharp in Vanity Fair. She gave new meaning to the term ruthless, but come on, 'fess up. Weren't you rooting for her just a little?
In many genres—particularly but not exclusively fantasies, thrillers, spy novels, crime stories, and mysteries—the protagonist is pitted against an antagonist. The antagonist can be a truly immoral or evil individual, such as Dr. Moriarty in the Sherlock Holmes stories, but he can also be a well-meaning but domineering parent or even a bumbling idiot who unintentionally stands in the way of the protagonist.
The bottom line is that the antagonist is at odds with the hero or heroine in the plot, and sometimes the story involves quite serious odds involving life-or-death circumstances. Shakespeare's Iago in Othello is a good example, but a protagonist can also be a whole group of people: the government, a cult, or a crime syndicate.
In some works, characters are created not as fully realized human beings or fantastical beings but rather as metaphors for a particular human quality. Lord Voldemort in the Harry Potter books isn't intended to be seen as a fully conceived person, but rather as a metaphor for the terrible outcome that results from scorning and opposing the power of love.
Characters As Plot Devices
In some cases, characters exist largely for the purpose of moving the story along from one plot point to the next. These characters are only sketchily conceived. They're flat characters—one- or two-dimensional. It's not who this guy is or how he feels but what he does that matters.
Stock characters can serve in this capacity. They tend to be stereotypes without much depth, like a womanizer at the bar or a beggar on a street corner, but they don't have to be. Stock characters can be more complex when the plot requires it.
Writers commonly create characters whose sole purpose is to motivate the protagonist to take the actions that propel the story forward. A good example of this type of flat character is Scar in The Lion King. Compare him to Simba, a round character. You knew Simba. Scar...maybe not so much.
Some stories are built around a time, a place, or a situation that requires certain types of characters to be present. These characters might not be terribly important to either the plot or the theme, but their absence would nonetheless be felt.
Imagine a story that takes place in a hotel environment without the inclusion of at least a few members of the hotel staff. A story that takes place on a spaceship headed for Mars would be incomplete without at least a sketch of the ship's captain, even if he's not a main character. Someone might be shot and killed during a bank heist. His identity, feelings, thoughts, and depth aren't important to the plot, but the fact that he was a fatality would be.
How to Create Characters
Be clear in your own mind about your character's purpose in your work before you begin writing and creating a character. Why and how does he move your plot to the finish line? You can begin fleshing him out when you've answered that question, and you'll probably want to give this part of the process a little time if he's your protagonist. Live with him for a few days or even a few weeks before you pen that first sentence. As events unfold in your life, ask yourself what he would do or how he would react in the same circumstance. Get to know him.
Although it's important to fully know and understand your protagonist's personality traits and her motivations, interests, and talents, you'll need far less detail for a character who simply serves as a plot device. You don't have to spin your wheels delving into what makes her tick.
Go With Your Gut
As anyone who has ever written a successful work of fiction will tell you, your gut is a powerful tool. And few if any drafts of fiction are perfect the first time around. More than likely, you'll dash out a rough draft then revise it two, maybe even three times.
If a character jumps into your pages seemingly out of nowhere while you're writing that first draft, why not let him hang out there for a while? Your subconscious might be trying to tell you something. He could be important later, providing a pivotal plot twist. You can leave him be and if he turns out to be superfluous, give him the ax when you're preparing your final draft. You can always write him out later if it turns out that he has nothing to offer.
No matter how significant or insignificant your character is, be sure the person is consistent and believable within your story's parameters. Motivations and actions must work together so that the reader isn't left confused and frustrated.