If plot is the skeleton — that vital framework upon which the rest of the body is built — then characters are the soul.
Characters are the reason we ultimately love or hate a story.
“I’m sick to death of the inarticulate hero,” said John Fowles. “To hell with the inarticulate.”
Characterization is in essence the depiction of those distinguishing characteristics that make a person individuated and distinct.
In real life, we are each defined by our actions and by what we say and do, and our actions are in turn defined by what we think. Thoughts shape actions.
In literature, that same basic thing is true: a character is shaped by his or her actions and words, and that is precisely why plot and dialogue are the sine-qua-non of character development.
But plot and dialogue are not the only tools we have at our disposal. Physical descriptions and narrative passages that tell the reader what the character is thinking are also important. Though neither of those two alone can completely flesh out a character, it is not true that you should never tell but always show. Narration exists for a reason.
Here, for example, is a short narrative description of one of the most unctuous and repugnant antagonists in all of literature: Pyotr Stepanovich, in the novel Demons, by Dostoevsky:
No one would call him bad-looking, but no one likes his face. His head is elongated towards the back and as if flattened on the sides, giving his face a sharp look. His forehead is high and narrow, but his features are small — eyes sharp, nose small and sharp, lips long and thin. The expression of his face is as if sickly, but it only seems so. He has a sort of dry crease on his cheeks and around his cheekbones, which makes him look as if he were recovering from a grave illness. And yet he is perfectly healthy and strong, and has never been ill.
He walks and moves hurriedly, and yet he is not hurrying anywhere. Nothing, it seems, can put him out of countenance; in any circumstance and in any society, he remains the same. There is great self-satisfaction in him, but he does not take the least note of himself.
He speaks rapidly, hurriedly, but at the same time self-confidently, and is never at a loss for words. His thoughts are calm, despite his hurried look, distinct and final — and that is especially noticeable. His enunciation is remarkably clear; his words spill out like big, uniform grains, always choice and always ready to be at your service. You like it at first, but later it will become repulsive, and precisely because of this all too clear enunciation, this string of every ready words, you somehow begin to imagine that the tongue in his mouth must be of some special form, somehow unusually long and thin, terribly red, and with an extremely sharp, constantly and involuntarily wriggling tip.
One page later in the same book, the reader is then treated to a narrative description of the protagonist — surely the damndest (anti)hero in all of literature:
But there are, it seems, such physiognomies as always, each time they appear, bring something new, which you have not noticed in them before, though you may have met them a hundred times previously. Apparently he was still the same as four years ago: as refined, as imposing, he entered as imposingly then, even almost as youthful. His faint smile was as officially benign and just as self-satisfied; his glance stern, thoughtful, and as if distracted…. Before, even though he had been considered a handsome man, his face had indeed “resembled a mask,” as certain vicious-tongue ladies of our society put it. Whereas now — now, I don’t know why, but he appeared to me, at very first sight, as decidedly, unquestionably handsome, so that it could in no way be said that his face resembled a mask. Was it because he had become a bit paler than before, and seemed to have lost some weight? Or was it perhaps some new thought that now shone in his eyes?
That, reader, is how you introduce with firepower your significant characters.