1. Read, read, read
I read many screenplays, plays, books, and short stories. Reading helps me get the feel for various writing formats and styles. I think of reading as the homework, with the actual writing as the final project.
2. Beg, borrow, and steal
This holds true for acting, too. Observe, observe, observe. Put your spy skills to use. Sit at a bus stop or the mall and observe those around you. Be a wallflower at a party, instead of participating in the conversation, listen and observe everything. In a high school acting class, we called these observations, character studies. We would watch for mannerisms, facial expressions, body language, and ‘bits.’ Acting bits are small stage moments a character does; i.e. how they might smoke a cigarette, run their hand through their hair when nervous or agitated, or biting their fingernails. We would write down what we observed and then ‘use’ what we learned in performing short scenes.
3. Listen to how people talk
Listen to conversations that take place around you every day. Not being nosy, but listen to a conversation at a restaurant or standing in line at a grocery store. The details are not important; listen to voice patterns, nuances, dialects, natural pauses, phrasing, inflections, anything that might help with dialogue and the way people talk. With that in mind, when writing, let go of your inner grammar geek—use fragments, we talk in fragments, use contractions, we speak in contractions; however, use stereotypes, profanity and slang sparingly. Consider the flow and rhythm of a real conversation. Characters can have unfinished sentences, be interrupted, and can argue--just like in real conversations.
4. Visualize the scene
Before I write down a single line, I visualize the scene. I see the setting, the time and place, I visualize the action, hear the characters voices in my head (yes, I hear voices), then I write what I saw and heard. Good dialogue will move the action forward and fleshes out the characters. Think of it as a road map.
5. Write it and then read it
It is best to do this standing in front of a wall, hearing your own voice vibrate. Does it sound natural? Does it sound forced? Is it interesting? Are you giving too much away? Are you telling or showing? Is your dialogue moving the scenes forward? Does it sound authentic?