Inspired by a response by Sam in another thread
Obviously the ideal state is for a novel to be both an excellent story and well written. But, let's pretend that's not possible.
Let's say somebody who is only barely competent as a writer happens to be a great storyteller. They're truly brilliant when it comes to narrative, character, everything that exists beyond the page, but the actual prose is straight journeyman, grade-school-level stuff.
Let's say another writer exists then, one who is a truly gifted writer but happens to only be an average sort of storyteller: That is, they can 'tell a story' to the extent they can deliver according to formula. There's nothing wrong with their writing, certainly nothing you could point to, but the ideas, characterization, concept is all pretty much in line with what is already out there. There's nothing that feels new, nothing interesting. However, again, the writing is REALLY good. That is, it's technically flawless, inventive, beautifully poetic at times.
Which of these two scenarios is more likely to yield success? Are either of them? Assume for the moment that it is not possible for either of these two individuals to improve dramatically... In essence, can fairly rough writing be overlooked by the average writer if there is a solid underlying concept...or are great stories completely enslaved to competency?
At what point does a great story become undermined by poor writing skill (if any)? At what point (if any) does great writing skill become undermined by an average or worse story?
Is it possible to be such a great storyteller that you will still be successful even if you can't even master basic grammar? Is it possible to be such a great writer that you will still be successful even if the actual things you are writing about are generally considered dull as ditchwater?
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