Think like a viewer:
No matter if you are writing a whole script or just a single scene, thinking like a viewer is the basic rule of script writing. Imagine yourself in the audience and then see what your character is saying.
Understand the situation:
Though it looks ridiculous, as being a writer who else would have a better idea than you. However, you should not ignore the importance of a review. Separate yourself from the story and try to find out how significant this scene is. This will really help you to write an influential stage whisper.
Choose your words carefully:
You should take great care of the language and words you use. You must understand your character before putting any words in his/her mouth. Remember, a naive character will speak in its own way and similarly a cultured character cannot use rough language even in the whisper.
Keep it short:
You must know that a whisper is never too long as others are also on stage and your characters cannot have a long conversation in front of them. You should be brief and concise as you need to hide and reveal at the same time. Your audience will listen to this whisper whereas the other characters will remain unaware of this suspicious activity.
Do not involve physical activity:
You do not have the margin to involve the physical activity in a stage whisper otherwise there will no sense in using this technique for the development of the story.
Write a first draft:
Now, when you are equipped with all the information, start writing the dialogues. However, you should be editing as well. The best thing is to read over and again. You must be a ruthless editor to formulate an influential stage whisper.