From author Rowena Macdonald via Glimmertrain:
Many writers say they find writing dialogue difficult, which I always find surprising, as, without wishing to sound self-aggrandizing, dialogue is the one aspect of writing I find easy. To me, it isn’t that impressive to find dialogue easy. After all, we are primarily verbal creatures, we are surrounded by conversation every day, and most of us spend more time watching films and TV than we do reading books. I am always far more impressed by writers who are able to craft complicated plots, for example, since this is an aspect of writing I find difficult. To my mind, plotting is a superior skill because it isn’t something that occurs in reality: events don’t pan out in a neat, compelling sequence, loose ends are not neatly tied up and much of life is mundane, unsymbolic and random.
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1. Read it aloud. If it doesn’t sound natural, it isn’t. Make sure it sounds different from prose. Remember, few people talk in complete sentences.
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4. Don’t write out “ums” and “ers.” They are realistic, but they look cartoonish in a piece of literature. Instead, use ellipses to give the impression of pauses or uncertainty. Ellipses can also be used at the start and end of dialogue, when someone has been talking for a while and is likely to go on awhile, to give the impression of the other characters tuning out.
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6. If writing dialogue for a character with a specific accent, don’t write it out phonetically, as this can look patronizing and old-fashioned. Use odd syntax and a few choice bits of slang to convey their accent.
Link to the rest at Glimmertrain
Here’s a link to Rowena Macdonald’s books