Character Voice - Hear them before you write
Each principal character in your novel or short story should be unique, and this distinctiveness is more than how they look. Every character should have a special voice—not just slang or dialect but a deeper presentation to reinforce their personality.
For example, if you create an insecure, twenty-something man, you might want include modern language and ensure he doesn’t have decades of experiences to draw upon. If he's recently out of college and a bit nervous about entering the career world (e.g., executive, detective, attorney, or national park ranger), he'd have physical traits that showcase his anxiety. Maybe he eyes the exits, has trembling hands, or glances at the ground. His conversations might include sounds to play for time, like "er" or "hmm." As he gains confidence in your novel, his tells and dialogue would mature along with him.
Internal thoughts can help to solidify a character’s voice. If you’ve read some of Andy Weir’s works (e.g., The Martian or Project Hail Mary), you'll know Mr. Weir is a master at creating highly intelligent, snarky, but very loveable characters. He makes use of a wide range of techniques to solidify a character’s voice, but one of the tools he uses especially well is internal thoughts—to magnify how the character feels about being in the middle of an outrageously difficult situation—like facing mortality or the Earth's demise.
Small descriptive words embellish a character’s voice. Someone who whispers might be timid or shy. A raspy tone can reflect a person’s health or age. Short declarative sentences with demands, can reflect an assertiveness or aggressiveness.
Dialogue pacing can emphasize personality traits like narcissism, arrogance, ignorance, loyalty, or cruelty.
Sentences/dialogue with passive-aggressive wording can reflect personality. For example (my parents were the masters at this....):
To get the other person to go to the store: “I’d go to the store for us, but I might buy things you don’t like.”
To encourage a young person to come home early: “I’d let you stay out late, but your father will be worried sick if you not home by 10:00.”
Know your characters well and be sure they talk, move, and think in ways consistent with their personality. If you write their dialogue and actions after you know them, their voices will shine through.