Visualize the play in your head. Take note of any lines that are critical to the plot and must be delivered in a certain way to match the tone of the scene.
Write the emotion the actor needs to use when delivering the line in parentheses in front of the line. For example, if the line needs to be angry, write "(angry)" before the line of dialogue.
Find the correct volume for delivery of the line. Write this in parentheses as well. If the line needs to be said in a whisper, type "(whisper)" before the line.
Stay in communication with the actors. The actors need to know your vision for the play and how you feel about their performances. This helps establish a rapport with the actors.
Direct your actors to stay in character. Ask them to study their characters. This helps the actors know how to deliver certain lines. Many times an actor who knows the character will deliver dialogue perfectly because he understands who the character is.
Listen to the actors as they deliver lines. Tell them immediately when something sounds wrong.
Give the actors examples of how the line needs to be delivered. Let them repeat the dialogue back to you a few times to make sure they understand the changes.
Expereiment with emphasis on the words in the dialogue to change how the dialogue affects the scene.