1. No two people talk exactly the same. In believable stories, as in life, each person will have his or her own rhythms of speech, pet phrases and regional or family expressions. This doesn’t mean each character should broadcast his geographic or cultural background with every sentence, however. It just means that if, by about a quarter of the way through the book, a reader can’t tell your characters apart merely based on their dialogue, you haven’t made each character’s "voice" distinctive. The important thing here is to be subtle when drawing those distinctions. If you’re not sure what this means or how to go about it, here’s an exercise to try: the next time you’re in a crowded, public place, pay attention to the bits and pieces of conversation floating all around you. Notice how different people express the same thoughts differently.
For example, where one person might say, "I called Sally," another might say, "I phoned Sally," or, "I rang Sally." Where Joe (in his forties) says, "That whole night was a waste of time," Jake (a twentysomething) might say, "Two words: epic fail," and Steve (an ex-military man) might say, "FUBAR, all the way, man." Thinking about your characters’ backgrounds, histories, and even biases and motivations when constructing their dialogue will help in making their voices distinct from one another.
2. Life is not a movie. While heated exchanges, adamant diatribes and weepy heart-to-hearts all have their bit to contribute in various stories, they should be used sparingly if you don’t want your novel to read like a soap opera script. If you’re prone to succumb to melodrama in your dialogue, try reading it aloud. If the words feel or sound unnatural coming out of your own mouth, they shouldn’t be coming out of your characters’ mouths, either. Of course there’s some wiggle room here if you’re writing something historical, a fantasy, sci-fi, or anything else with purposely unusual language.