From Now Novel:
Whether you’re writing a novel or a short story, you are going to want to make your dialogue natural and true to real life, as it’s spoken in the real world. How do you go about achieving this, when ‘natural dialogue’ can be boring to read if you write it verbatim?
As to why you should make your dialogue sound natural, Daniel Boyko and Zoha Arif of Polyphonic Lit have this to say:
Unnatural, inorganic dialogue can make any character sound like an evil robotic Martian stranded on the great abomination of earth (with no hope of reconnecting with its extraterrestrial kind), trying to camouflage into human society and failing completely and utterly to do so.
. . . .
In his book How to Write Dazzling Dialogue, James Scott Bell explains the difference between real speech and fictional dialogue. He writes that:
Dialogue is not real-life speech. It is stylized speech for which the author, through the characters, has a purpose.
That’s a crucial distinction. We don’t want to merely capture reality in our fiction. We aren’t filming a documentary.
What we do is render something that feels real but is intended to create a desired effect.
Real-life speech is meandering and often boring.
Fictional speech doesn’t meander (unless, of course, a character has a strong reason to run on and on).
Firstly, you have to listen to natural dialogue. The best way of doing this is not to have a conversation, because then you are so busy being part of the give and take that you don’t really hear the natural rhythms of dialogue. One recommendation is to go to a coffee shop and eavesdrop. Listen to how people speak in the real world, note the rhythms and cadences: when people get excited, or sad, note how the tone changes. Note, too, how people don’t use complete sentences, how people pause, and sometimes the listener will rush in to fill that gap while the speaker is pausing. Notice when people lose their place, forget things, and just simply don’t complete their thoughts. And yet despite that, the meaning of what they are saying is implied by the rest of the sentence.
Link to the rest at Now Novel