If you are writing dialogue into your story, it should sound like real people actually talking to one another. Say the words aloud – if it feels awkward to say, it probably is.
Your character’s way of speaking should sound like a person of their class, background, ethnicity and style. That doesn’t mean that every word (or even every sentence) needs to be personalized to the point that the words become unreadable, but insert a word, phrase or accent every so often that is unique to that character
Think of a popular TV show or movie and the catch phrases that were always associated with a particular individual, like Joey Tribbiani in Friends and “How’re you doing?”, or Ricky Ricardo in I Love Lucy and “Lucy, I’m home.” Let your readers identify the person speaking by their style and not just “so and so said”.
If the character is uneducated, or crude, or drunk then they should sound different from the person who is well educated, prim and proper, and possibly snobbish. If they have an accent remember that it would probably be too confusing for your readers to decipher every word, but throwing in the occasional “accented” word will get the point across.
You can even use an occasional foreign word, but try to keep it simple so that your readers are not confused.
“Si senor, I do know that yoo can not see…”or “Sir, I can tell that it is not in your view…”or “Hey dude, ya ain’t seeing it…”
The same sentiment, but as you read those three lines, didn’t you get a vision of a different person each time?
Your narrative should be short and simple as well. Don’t use a thesaurus to write just so you can use multiple words to say the same thing. If it doesn’t sound natural, it will likely distract your reader.
You can add expression to your words by using descriptions rather than excess adjectives or needless exclamation points.
“Are you in here?” He whispered through the open door. or “Are you in here?” He yelled as he kicked the door open. or “Are you in here?” He spoke haltingly as he looked around the room.
Writing exercise: Write a few paragraphs about any subject. Now see if you can re-write each paragraph with no more than one to two sentences (not endless run-on sentences, please). Include all of the pertinent information by cutting out unnecessary descriptive terms and redundancy. Although your final words may sound dry because you are so limited, it is a good way to learn to tighten your thoughts.
Writing prompt: Describe the room you are in and your actions from your point of view (first person: I see, I hear…); then describe the room from the viewpoint of someone standing in the doorway and watching YOU (third person: She sat rocking in the chair…) Remember that in third-person the person describing the room cannot be in YOUR head, they cannot know HOW you feel.