Despite what you’ve been told, eavesdropping is a writer’s most valuable resource. For several days in a row write down a conversation that you overhear, without describing the people who are speaking. Use your ears, not your eyes, to imagine the speaker. Practice trying to remember phrases that are used. As you write them down, try to retain the personality in the words, the flow, and the purpose of the conversation, even if you have to make it up.