Conversational Speech: Dialogue should be kept simple, natural, and conversational. However, don’t use the exact words a person would actually speak because, in normal conversation, a person uses far more words than are needed. Actual speech needs to be whittled down, so it is crisp and clear. Never let your characters ramble.
Once you have a detailed character sketch of your main character, you will know how he will react in certain instances. You will be aware of his feelings, ideas, and beliefs. His personality will come through in his speech. He will help you write the dialogue, because, if you know him, you will know what he will say. As your reader gets to know your main character, he knows what the character will say, too. If your main person acts out of character, the reader will know and will feel something is wrong. Also, be careful not to contrive your character’s speech or have him preach. The reader doesn’t want to be talked down to—either by you or the main character. We teach the reader lessons by what the main character learns, not by lecturing.
These suggestions also apply to the minor characters to a lesser degree. When you write a novel, the minor characters are more developed, and these points become more relevant to them. In a short story, however, you don’t have much room to develop more than one or two characters with any depth. Usually the reader will identify with the main character, so he becomes your vehicle for getting your message across. The most powerful way to accomplish that is through his actual spoken words.