There is no such thing as a perfect person. Human beings are born flawed, some more than others, and as we grow, those flaws can become so amplified as to impact how an individual thinks, acts, speaks and believes.
Some of us wear glasses, some of us lisp, some are born deaf and some are mentally/physically challenged. These deficits vary in intensity, from being a mild irritant to a severe impediment. Many of us function quite nicely in society, while others of us need assistance to get by.
Your characters need to reflect reality in order to be believable. Imagine a scene in which too practically perfect people sit down at a coffee shop. Write the dialogue that they have. What do they talk about? How do they react? Do they simply sit and smile and nod complacently when the other speaks, or is there a true conversation in which opinions are stated? All too often perfect people are portrayed as the ideal. They live in huge houses, dress in the best fashions and drive the most expensive cars. What many of us can’t imagine are the worries or concerns that they have. Not that they don’t worry about illness or financial ruin, but what about the everyday nuisances that fill many of our lives? So have them sip from their expensive coffees and talk.
Reread what you have written. Does the scene feel true to life or is it stilted and flat?
Now give those two individuals characteristics that make them unique. Does one of them stutter? Blink obsessively? Twirl her shoulder-length hair? Tap his foot impatiently? Constantly check her watch or cell phone while attempting to pay attention? Move his hands about to emphasize points or sit with hands neatly folded in the lap?
Think about their physical appearance. Perhaps one of them has wind-blown hair while the other has a bad comb over. Maybe one wears black lipstick while the other one’s face is heavily caked with makeup. One might be tall with broad shoulders while the other is short and paunchy. One walks with a bit of a limp after suffering through knee-replacement surgery while the other moves about on the balls of his feet.
Be careful, however, when giving descriptions of characters. These details need to be worked slowly into the story, not delivered as a list. They are important, however, as those quirks impact how the character thinks and reacts.
So, how do these factors influence the conversation? The foot-tapping person might be abrupt in speech and impatient when listening to long-winded stories. He might interrupt frequently or finish the sentences of the other in an attempt to move things along. The paunchy individual might speak with a full mouth and sound muffled and indistinct. The hand-flapper might be somewhat clumsy and so inadvertently spill his drink. The bad hair individual might constantly be messing with it, trying to smooth out the tangles, and so lose concentration and need to have things repeated.
Now rewrite your scene, this time with your two imperfect people. Begin when one of them comes walking into the café while the other is already seated. How do they greet each other? What topics of conversation do they cover and how are these issues brought up? Make sure that your characters have unique voice and patterns of speech.
Is there a difference between your two versions? Which one feels more realistic? Believable? More natural?
Keep this exercise in mind each time you write.