Industry expert Donald Maass has studied the subject, being a writer and literary agent himself, and has answers to these questions.
You'll find those answers in his nonfiction book on the craft of Writing 21st Century Fiction from Writer's Digest Books, themselves an industry leader in all things related to writing.
I always learn something when I pick up a nonfiction writing craft book, whether about plotting or characters or dialogue or setting or description or comma placement (hey, it's happened before).
Rarely have I ever picked up a writing craft book that I so immediately and completely connected with and understood implicitly as being packed with guidance and wisdom absolutely crucial to improving my writing.
Writing 21st Century Fiction is an example of the latter.
I inhaled the book from cover to cover. Every chapter offers practical tips on how to transform your novel from good to breathtakingly compelling. To illustrate his points, Maass includes excerpts from actual novels, already on the market and spectacular in their own respective rights and categories, making it easy to comprehend what he recommends.
And he's quite forthright about his intentions. His suggestions are designed to help make authors' novels the best they can possibly be, but the tips are, in the end, just that: suggestions. He makes nothing a requirement or demand, and yet by the time I was finished reading, I saw the wisdom of nearly everything he recommended.
If you write fiction in any genre, whether commercial or literary, there are probably fewer than five writing craft books I would require you to read (on the off-chance that you were, say, a creative writing major in my graduate-level course on the subject). Writing 21st Century is one of the ones that makes the short list, in my admiring and enthusiastic opinion.
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Author: Donald Maass
Title: Writing 21st Century Fiction: High Impact Techniques for Exceptional Storytelling
Publisher: Writer's Digest Books
Purchase here: http://amzn.to/1XIvJRm
Disclaimer: The opinions I have expressed are my own.