Writing dialogue

  Dialogue should have one of these two functions:
1) Reveal character.
2) Move the story forward.
This is what Catherine Ann Jones states in her book "The Way of Story".
 When I edit my stories I keep those functions in my mind. I ask myself, " Do these words have a purpose?" I get rid of unnecessary chatter. I also read the dialogues aloud to know if they sound natural and realistic.

Dialogues can help us  show the reader something about the character. They can also contribute to shed light on the theme in subtle ways. Words sometimes lead to something that is not expressed but can be felt or intuited.
Catherine Ann Jones mentions that we can use dialogue to offer some back story about the characters. She sets Shakespeare as an example of this. Interestingly, I'd read some advice against using dialogue to reveal back story. I don't take this piece of advice seriously. The best lessons in writing are always learned by reading other writers' work.
  What are the challenges you face when you write or edit dialogues in your stories?