If readers don’t buy into your characters, they won’t buy into your story. Characterization is how a writer interjects the strengths, weaknesses, flaws, and all the other elements, personality traits, and behavioral peccadillos that make humans human, into their characters. E. M. Forster first distinguished these more 
fully developed characters as “round” and distinguished them from all the others, which he labeled as “flat.” Not all characters in a story have to be round, but the most important ones must be. The greater the burden you give a character to carry the story, the greater the need to more fully characterize him or her.
Round characters, said Forster, have the ability to surprise the reader. Flat characters are predictable and thus stereotypical. They are consistent and have no complexity. They will make your story boring because they are boring. Round characters are capable of interjecting the tension that makes fiction what it is. We go through life trying to keep our lives on an even keel. We like books where the lives of the characters are anything but.
Unfortunately, many writers merely stop the story and physically interject narrative description, thinking they are adding characterization. Characterization is not a spice to be sprinkled over the top of your narrative or, like spackle, putty-knifed into the cracks between the action and dialogue. Characterization consists of threads woven into the fabric of your story.
Elements of characterization can be determined by (1) a character’s actions, (2) his or her statements, (3) what s/he thinks or feels (internal thoughts) (4) what others think or have to say about him or her, or (5) some combination of the above. Characterization may consist of pure physical attributes or deep mental analysis—from the heroine’s button nose or her quirky preference for the chemicals she used to poison a string of wealthy husbands.
How characterization is interjected into story depends in part on the story’s voice. A first person narrative lets the reader into the main character’s (narrator’s) mind. The first person narrator would likely supply much of the characterization of the other characters, although the dialogue and interaction of the characters could help provide alternative sources of insight into their personalities. In the third person narrative, the available tools of characterization expands, as would the potential depth of the resulting characterization, depending on the latitude the writer gives to the narrator, of course.
Characterization can range from the physical attributes of a person to their deepest and most intimate of thoughts. Whatever elements of character are presented, they need to be relevant to the content of the location within the story where they are placed. In a police story, the sergeant may well open a file and rattle off characterization details of a criminal from the folder. But if you stop your romance story and essentially do the same thing, it will come across as forced and jarring, like hitting a pothole on an otherwise smooth road. As important as characterization is, the technique of injecting it into you story in the right place and right way is as important. The alternatives need to be carefully considered.
Jim had chubby cheeks is descriptive, but The frigid night wind chilled Jim’s chubby cheeks and he pulled his scarf up to the bottom of his thick glasses works better to give us an idea of Jim’s physical attributes while staying within the story. Before you can interject such description, however, you have to have a pretty good idea of the character traits of your key characters. You can sit down and create a sort of personality profile of each character, or build a dossier as your story proceeds and you put more and more life into your characters.
You can imagine how complete the character traits file for Kinsey Millhone, Sue Grafton’s female gumshoe in her “Alphabet” detective series, became as she approached her twenty-sixth volume. Apparently, Kinsey was not fully formed in the beginning. As Grafton writes in Kinsey and Me (2013), the detective “entered [her] life, like and apparition, sometime in 1977.” F. Scott Fitzgerald allegedly detailed his chapters right down to the number of words he would use, so likely he had a very good idea about his characterizations before he wrote a single word in an actual chapter. I tend to let my characters take shape as I write. That means I have to go back and flesh them out more fully later, but I feel I have more flexibility and control over the structure of the story doing it this way. Soon enough, as Grafton says about Millhone, your character will be “peering over [your] shoulder, whispering nudging [you], making bawdy remarks.” Mine have told me when they wouldn’t do something the way I was suggesting they would.
There is no perfect way or technique of characterization, i.e., formula. You find what works for you. But you must contemplate characterization as you write and as you think about what you have written and as you think about what you’re going to write next.
I think Harper Lee is the master of characterization, and for that reason I’ve included a lengthy example from To Kill a Mockingbird to show how effectively characterization can be interjected into the story, becomes part of it, and, as a result, draws the reader into the story.
Jem and his sister Scout and their next-door friend Dill don’t run around on the pages of the novel. They pull you into them. You don’t see mere characters but dear friends with whom to sympathize and empathize. While their father Atticus was out of town, Calpurnia, their black housekeeper, took Jem and Scout to her church on Sunday morning. Scout, the narrator, reports:
Slowly, painfully, the ten dollars was collected. The door was opened, and the gust of warm air revived us. Zeebo lined On Jordan’s Stormy Banks, and church was over.
I wanted to stay and explore, but Calpurnia propelled me up the aisle ahead of her. At the church door, while she paused to talk with Zeebo and his family, Jem and I chatted with Reverend Sykes. I was bursting with questions, but decided I would wait and let Calpurnia answer them.
“We were ‘specially glad to have you all here,” said Reverend Sykes. “This church has no better friend than your daddy.”
My curiosity burst: “Why were you all takin’ up collection for Tom Robinson’s wife?”
“Didn’t you hear why?” asked Reverend Sykes. “Helen’s got three little ‘uns and she can’t go out to work—“
“Why can’t she take ‘em with her, Reverend?” I asked. It was customary for field Negroes with tiny children to deposit them in whatever shade there was while their parents worked—usually the babies sat in the shade between two rows of cotton. Those unable to sit were strapped papoose-style on their mothers’ backs, or resided in extra cotton bags.
Reverend Sykes hesitated. “To tell you the truth, Miss Jean Louise, Helen’s finding it hard to get work these days . . . when it’s picking time, I think Mr. Link Deas’ll take her.”
“Why not, Reverend?”
Before he could answer, I felt Calpurnia’s hand on my shoulder. At its pressure I said, “We thank you for lettin’ us come.” Jem echoed me, and we made our way homeward.
. . .
Jem said it looked like they could save the collection money for a year and get some hymn-books.
Calpurnia laughed. “Wouldn’t do any good,” she said. “They can’t read.”
“Can’t read?” I asked. “All those folks?”
“That’s right,” Calpurnia nodded. “Can’t but about four folks in First Purchase read . . . I’m one of ‘em.”
“Where’d you go to school, Cal?” asked Jem.
“Nowhere. Let’s see now, who taught me my letters? It was Miss Maudie Atkinson’s aunt, old Miss Buford—“
“Are you that old?”
“I’m older than Mr. Finch, even.” Calpurnia grinned. “Not sure how much, though. We started rememberin’ one time, trying to figure out how old I was–I can remember back just a few years more’n than he can, so I’m not much older when you take off the fact that men can’t remember as well as women.”
“What’s your birthday, Cal?”
“I just have it on Christmas, it’s easier to remember that way–don’t have a real birthday.”
“But Cal,” Jem protested, “you don’t look even near as old as Atticus.”
“Colored folks don’t show their ages so fast,” she said.
“Maybe because they can’t read. Cal, did you teach Zeebo?”
“Yeah, Mister Jem. There wasn’t a school even when he was boy. I made him learn, though.”
Zeebo was Calpurnia’s eldest son. If I had ever thought about it, I would have known that Calpurnia was of mature years—Zeebo had half-grown children—but then I had never thought about it.
“Did you teach him out of a primer, like us?” I asked.
“No, I made him get a page of the Bible every day, and there was a book Miss Buford taught me out of—bet you don’t know where I got it,” she said.
We didn’t know.
Calpurnia said, “Your Grandaddy Finch gave it to me.”
“Were you from the Landing?” Jem asked. “You never told us that.”
“I certainly am, Mister Jem. Grew up down there between the Buford Place and the Landin’. I’ve spent all my days workin’ for the Finches or the Bufords, an’ I moved to Maycomb when your daddy and your momma were married.”
“What was the book, Cal?” I asked.
“Blackstone’s Commentaries.”
Jem was thunderstruck. “You mean you taught Zeebo out of that?”
“Why yes sir, Mister Jem.” Calpurnia timidly put her fingers to her mouth. “They were the only books I had. Your grandaddy said Mr. Blackstone wrote fine English—“
“That’s why you don’t talk like the rest of ‘em,” said Jem.
“The rest of who?”
“Rest of the colored folks. Cal, but you talked like they did in church . . .”
That Calpurnia lead a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages
In those 767 words, Lee didn’t stop the story to provide characterization in all of the ways it can be done. The characterization is woven into the scene as it unfolds. The reader might as well be walking right along with them as a silent witness. Characterization flows from the conversation and the subjects of the conversation. Lee subtly interjects descriptions that characterize the people in the story. We learn about the black church, the level of illiteracy, the harshness of the fieldwork, and the family burdens on the black population who had no household support—their babies went to work with them. We learn the source of Calpurnia’s ability to read, but in doing so obtain insight into the Finch family and their philosophies on race in those Depression days in the Deep South. We learn that Cal leads a double life, probably so not to be considered uppity by her own folk. The characterization is woven into the questions of an ever-curious Scout and is organic to the larger landscape of the book; it’s part of the fabric of the story.
The lesson is to have your characterization come from the story and not be a coating brushed over it.
Another example: You could write, The soles of Dad’s shoes were wearing thin. He lamented he could step on a piece of gum and tell you what flavor it was. / OR /Dad eased himself down into the old, battered easy chair, and slowly slipped off his shoes and rubbed the bottoms of his feet. I could see mom would have some darning to do. The oval patch on the right shoe’s sole looked like a knothole in an old gray board. When Dad stuck his hand inside and pushed his finger against bottom of the shoe, the paper-thin leather gave way and his finger stuck out. He sighed, a tired sigh. “Well, that settles it. I can now step on a piece of gum and tell you what flavor it is.” He looked up and over and gave me a wink. “Do we have a piece of cardboard lyin’ about, Mother?” he asked. / Mother came out of the kitchen where she was making bread with the back of an old writing tablet in one hand a pair of ancient scissors in the other, and like a surgeon cut out a piece shaped like a shoe sole and handed it to him. “Bout time to dig deep into the cookie jar and get a new pair or some repairs,” she said.
We learn facts, but also information from which we can make assumptions about the characters: A hardworking father, home and tired; a poor family, as witnessed by the father’s shoes and confirmed by his wife’s temporary repair. Not unlike Mockingbird, we’re getting an eyewitness account from a child’s perspective and from a natural family interaction that doesn’t come across as a plant-on device. Writer Sol Stein would call these little identifiers of characterization “markers.”
In his 1995 book, Stein on Writing, he describes these markers as “easily identified signals that to the majority of readers will reveal a character’s cultural and social background.” They are small references that likely will connect with or create in the reader’s mind an image that enhances the detail of what has been said or described. These markers provide insights into character through the use of hints of traits, actions, and interactions that can be direct or indirect, blatant or subtle. The markers usually deal with contrasting cultural and social characterizations that enhance a reader’s own understandings but also create tensions, which he divides into “upper class” and “lower class.” A woman with curlers under a scarf in public would connote lower class. What does the distinction between a woman with “claw like fingernails and excessive rouge” and one with manicured nails and beauty-shop perfect hairdo tell you about which class they belong, and thus their character? Or, what does a man with black under his fingernails wearing a suit tell you about him? Someone incessantly chewing gum? A woman in cheap clothes and gaudy jewelry? A person’s car, choice in beverage or food, and mannerisms can all serve as markers, explains Stein.
I agree with the technique, but Stein’s examples seem to rely on stereotypes and too much of that makes one’s writing hackneyed. So be careful.
Although not as complete as Mark Twain, Harper Lee also uses language as characterization markers. Scout picks up on Calpurnia’s duo languages and Calpurnia explains not only their sources but also why she speaks differently depending on where she is—her church versus the Finch residence. Think what her different behaviors tell you about Calpurnia and her people. Some people like to show off their differences. Calpurnia hides hers.
Stein expresses a preference for “action markers” or “. . . sentences that describe what a character does and at the same time reveal something about the character’s upbringing or background.” He presents three sentences with quite different “instant characterizations” from the same setting, a restaurant:
1) Every time Zelda ate in a restaurant, she found some reason to send food back to the kitchen.
2) Louis always played it safe by over tipping the waiter.
3) As usual, Angelica let her food get cold because she was busy watching everyone else in the restaurant.
Stein mixes the concepts of characterization with the concept of conflict. By showing dichotomous character traits, a writer enhances the contrasts between and among characters and thus increases the tension with a story. Stein goes so far as to state: “If you are presently writing a novel, have you examined it to see if there are some social or class differences between your two most important characters? How do those differences influence the story? If you have neglected such differences, how might you bolster your story by adding some social and cultural differences that arouse emotion?”
Stein links characterization to conflicts that create the tensions needed for that magical ride up from the inciting incident to the climax. At first, it might seem he is mixing characterization with other aspects of fiction, but he is actually telling us about the weave of characterization and story. What Stein is saying is that sharp differences between your characters is a source of characterization that also builds the tension that makes fiction all the more powerful.
You won’t likely find a lot of definitive discussion about characterization, mostly lists of its elements. I suggest you read, or re-read, Mockingbird. Were you to highlight the entries where Lee presents characterization in its various forms, you’ll likely dry out a highlighter, maybe two.
Suppose that’s something the Pulitzer committee was drawn to?