From Writers Helping Writers:
Screenwriters are masters of dialogue. They rarely have the opportunity to include a character’s innermost thoughts on the screen so they rely heavily on dialogue to drive the story forward, develop characters and convey a range of emotions. By studying the art of dialogue through reading screenplays and watching movies or TV shows, it will help you develop your own characters and stories.
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There’s an array of movie and TV scripts available on the internet for you to read and I recommend you start with the screenplay of one of your favorite movies. I will add that screenplays available on the internet are not pirated, as screenwriters and film production companies often make them available for the public to read after a movie or TV show has been produced.
When studying dialogue, here are some points for you to consider:
What Isn’t Said
Humans rarely say everything we’re thinking and feeling and neither should your characters. If we’re talking about something that scares us or we’re in danger of being found out or simply too embarrassed to talk about a subject, we change topics or do something that helps us avoid talking about something we don’t want.
The Coen Brothers are brilliant at holding back dialogue that creates tension so that when a character does speak, we’re mesmerized by their words and really want to know what they have to say. The movie No Country for Old Men is a great example.
No Two Characters Should Sound the Same
The way in which a character speaks is a culmination of their experience, upbringing and beliefs and no two people should ever sound the same. Listen to the way your friends and family talk. People have favorite words and expressions, some interrupt conversations while others sit quietly and wait until they’re asked a question or think a long time before saying how they feel. Others avoid talking about their emotions all together. Imagine a conversation between a teenager and someone in their mid-forties. They’re likely to use different idioms and expressions the other may not understand.
Look at each of your characters and figure out what kind of person they are. Are they a leader, follower, questioner, peacemaker or a troublemaker? How would this be reflected in the way they speak? Their traits will greatly influence their conversations with others.
Read the Dialogue Out Loud
The best way to discover if dialogue is working is to read it out loud. You can do it yourself or enlist a friend or family member to be the other character or you can use one of the many available reading programs that will read what’s on the page to you. Does the dialogue sound natural or stilted? Are they using the other character’s name too much in the dialogue (a mistake nearly every writer does!)? Are they too wordy? Remember, most conversations between people are short and simple. Most of us don’t use big words and opt for the simpler version to get our message across. We also don’t speak for great lengths of time without being interrupted and neither should your character.
Link to the rest at Writers Helping Writers