Dialogue is the most powerful tool a writer has for projecting character.
When The Sun Also Rises was published by Hemingway in 1926, many critics took the young author to task for using dialogue so freely in the novel. The novel made Hemingway an international literary sensation. The criticism had vanished by the time his second novel, A Farewell to Arms, came out in 1929. The critics realized Hemingway knew exactly what he was doing.
And what was he doing by using so much dialogue? He was characterizing his characters. He was providing the reader with necessary information. He was moving the narrative forward; and he was keeping different levels of tension in the narrative.
All of these things made the story enjoyable for the reader and kept the reader reading. Papa knew exactly what he was doing.
This is what you want to do in your novel, play, short story and flash fiction. Writing a narrative is about all these things.
Luckily for the flash fiction writer, this only has to be done for a few hundred words. So, yes, a flash fiction story can be nearly all dialogue if you keep in mind what Hemingway did with his dialogue.
Cover of A Farewell To Arms
Just let me give you a useful writing tip. Make it easy on yourself and use the first few words of your flash fiction story to let the reader know the locale. If you do that, you can pretty much work everything else the reader needs to know into the dialogue.
Don’t have your characters give speeches for the benefit of the reader. Have them talk to each other. And remember, since they know about one another and about what is going on, they are not going to say everything that’s on their minds.
That’s where you come in. What they say and what they do not say has to imply what they mean.
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