Avoid one-dimensional characters. The more complex your characters, the more they come alive for the reader. Look to psychologists to find ways to flesh out your characters. The Enneagram personality test and the Myers-Briggs paradigm both offer insight into the complexities of the human personality.
Make each character distinct. Writers can get so involved with plot that they don't realize all their characters have the same look, the same speaking style and the same personality. The characters might even have similar names. Readers will quickly give up on a story populated by interchangeable characters named Mike, Mark and Mack.
Give your protagonist some flaws. For example, she should have moments of self-doubt or occasionally exhibit poor judgment. Likewise, antagonists should not be painted as all bad. Give them a few sympathetic qualities. Perfectly good and perfectly bad characters are unbelievable and boring.
Use the actions of secondary characters to complicate the protagonist's main problem. Secondary characters might also have their own subplots, but those subplots should relate to the main story line.
Create suspense and tension by continuing to give your protagonist more and more problems that stand in the way of her goal. To make the character's difficulties believable, weave some of her past experiences through the plot. It shows readers why she acts as she does at the time of your story.
Create dialogue that develops your characters by showing who they are. The way they speak gives information about their educational level, their prejudices and their motives. Characters often reveal their truest personalities when they are stressed. Just as people do in real life, characters might accidentally blurt out things they didn't mean to say.
Avoid writing all your dialogue in full sentences. It will sound phony, because that's not the way people talk. Rewrite conversations where characters are stuck in a string of questions and answers. Your readers will think they are eavesdropping on an interview, and the style is monotonous. Instead, have one character anticipate a question and answer it before it is asked. Sprinkling dialogue with non sequiturs also makes characters sound natural.
Avoid overuse of tags other than "said." Inexperienced writers often think characters are more interesting if they exclaim, murmur or mutter. "Said" is unobtrusive, however. Because it doesn't call attention to itself, the reader's focus stays on the dialogue.
Write dialect with care. Give just enough of the distinctive pronunciations to make it identifiable and maintain the speaker's voice. When overdone, dialect can make dialogue almost unintelligible, and frustrated readers are likely to give up within a few pages.