Choose a scene you want to write. This can either be something new for a practice exercise or a scene from your screenplay in progress. Identify the characters along with their respective objectives in the conversation.
Examples:
Person A wants to invite Person B on a date.
Person A is fleeing from danger, unaware that Person B is the real threat.
Person A is confessing a crime to Person B, a priest.
Person A is expressing anger toward his adoptive parent, Person B.
Consider the length of the scene. For the purposes of this exercise, let's say the conversation is confined to no more than two minutes. In a properly formatted screenplay, that translates to two typed pages and should teach you the value of making every line count toward revealing layers of character, imparting crucial backstory and/or contributing to the escalation or resolution of the core conflict. If a conversation doesn't accomplish any of these goals, it shouldn't exist.
Identify elements that will vocally distinguish your characters from each another. For new writers, there's a tendency to give everyone the same voice, regardless of gender, age, ethnicity, social status, occupation or level of education. An older, sophisticated Englishman, for example, won't have the same vocabulary and communication skills as a ditzy teenage girl, a gang member from an urban neighborhood or a trucker born in the Louisiana bayou. (See Tips.)
Recruit friends to improvise dialogue scenes with you before you write them. Explain what you want the conversation to accomplish, turn on a tape recorder and set a timer. Example:
"You're the headmaster of a private school who has just turned down my son's application. I believe it's because you're prejudiced and want you to reconsider your decision."
Repeat this exercise until you can deliver the desired content within two minutes. Transcribe this session, review it and see if you can edit it to an even tighter focus by eliminating extraneous and/or repetitive chatter.
Determine the value of subtext in your dialogue as well as instances where action -- or silence -- could convey more than words. Examples of this are characters who avoid confrontation by answering every question with a question; characters who exert dominance by interrupting, shouting or lecturing; characters who are insecure and speak very little; and characters who are too scattered to maintain a consistent thread of conversation. Consider, too, whether your characters are sitting, standing, eating, walking or engaging in strenuous activity during the exchange, as these factors will influence how they talk.
Read your dialogue out loud or, better yet, use readers that aren't familiar with your plot or characters. Oftentimes what looks perfectly fine in print sounds silly when spoken aloud. Example: "I have just discovered something moving on Uranus." Likewise, a line could contain tongue-twisters or be so long-winded that it can't comfortably be delivered in one breath.