Realistic characters are multidimensional and have good and bad qualities, like real people. Don't make your hero completely good and your villain completely bad; the story will sound false and be less compelling. In addition, avoid basing characters on ideas or stereotypes. Instead, give all characters individual characteristics, flaws and strengths, and make sure characters act in a way that makes sense given their personality and background.
Writing good dialogue is tricky, but poorly written or stilted dialogue immediately ruins a story. Read your dialogue aloud to make sure it sounds natural, and don't try to write dialects or accents that are unfamiliar to you. Listening and transcribing everyday conversations can help you capture the rhythm and cadence of spoken language. In addition, don't try to provide too much information through dialogue; conversations that include too much information don't sound natural.
If you're writing about a location, time period or situation unfamiliar to you, do your research before you start writing. For example, make sure you portray the culture and weather of a place accurately, depict accurate technology and culture in fiction set in the past and don't write about unfamiliar medical or legal issues without researching first. Writing about places and situations you know can help you ensure that your details are realistic and authentic.
Realistic fiction requires a plot that is believable and compelling. Characters might battle their own fears and insecurities instead of an external antagonist, but some struggle must occur to make your story compelling. Include elements of foreshadowing so twists and turns in the plot don't feel too sudden or abrupt; the resolution of a problem should feel natural because the story has led up to that point. In addition, make sure your characters' goals and actions fit with their personality. Let your characters drive the plot instead of conforming them to fit a storyline.