The same notion holds true with writing. Story ideas are a dime a dozen. I have a notebook of story ideas to prove it. I'm sure you have lots of great ideas too, as do many people who've never written a book. But obviously, not every idea turns into a book, or most of the people in the world would be authors.
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This point was driven home to me lately because, for the past few months, I've been working on the first draft of my sixth novel. You'd think that by now, writing a novel would have gotten easier for me. Not so! This might have been the most difficult first draft that I've ever written. The story idea seemed awesome when I first set out to write it (and I still like the premise very much), but the idea introduced a restriction that I wasn't aware of in the beginning. I won't reveal what that restriction is because it would spoil the story, but it's caused me a ton of headaches during the course of writing the manuscript.
I'm confident that I'll eventually whip this story into shape, but the experience just reminds me that a great idea does not equal a great book without the execution and hard work to back it up. Stories don't write themselves, no matter how many novels you've already written!