Use descriptive language suitable to each character speaking in narrative. Apply a vocabulary that the character would use regularly. Employ vivid words -- within his vocabulary -- to describe people, places and situations. Avoid pretentious language when the character is uneducated, for example. Stay away from using generic or neutral language at all times, as well. Generic or neutral language employs words anyone could say at anytime and mean the same thing. Using dynamic, character-specific language enhances the fullness of the character himself and boosts the appeal of the narrative.
Properly pace the dialogue. Paint pictures with the words, but keep the momentum moving forward. Pick certain spots to slow down and indulge with detail and other sections to speed over nuance and give the viewer (or reader) only blunt, forceful pieces of information. Balance the pacing so a steady rhythm develops making dialogue fluid -- no section should be paced too fast or too slow.
Balance the exposition. Exposition in dialogue is educative -- it tells the viewer (or reader) something about a person, place or thing. Use dialogue that tells the viewer (or reader) something new and/or illuminates a previous piece of information at all times. Position how and where you use exposition carefully -- to avoid making the story flat and boring. Avoid forcing exposition -- retelling exposition -- for fear that the audience will not comprehend what is going on.