Writing is telepathy. It is trying to communicate a message or image (both, really) from your mind to another person's mind. No person will see exactly what your mind sees. It is the job of the writer to communicate as much as what is necessary, while leaving enough left out for the reader to have fun filling the gaps. The writer cannot be fully detailed, or the work becomes an instruction manual.
This King's first piece of advice in his book On Writing (after forty pages of biography). Therefore, we can presume that this is his most important message. I almagamated this from his three page introduction to 'how to write'. He uses the example of a magician standing over a bunny in a cage, with the number 8 on his back. This is all important information, and all the writer basically needs to communicate, but there are still gaps. What kind of cage is it? Glass or wire-frame? How big is it? What kind of bunny? Is it a Houdini-type magician or Siegfried and Roy type magician. None of these are important to the message the writer is trying to communicate, and it's left to the reader to fill in the gaps with the wire-frame cage and Houdini-looking magician. This is why reading is fun, the reader takes an active role in forming the story and image in his mind. So the writer can't be so detailed that the message and image is lost in the nuances of the image.
I like the idea that writing isn't just communicating like a letter or an anecdote. It's mind to mind. It's taking what's in your mind and being able to put it in someone else's. The medium has to be something physical though, like words on paper, because we're simply not that good. If you could do it mind to mind with no middleman, I'm not sure you could get a coherent story out of it.
It's also like playing "Telephone". No one is ever going to get the same thing out of it that you put into it. You're never going to be able to communicate the message the same way it was in your head. No one's ever going to interpret it the same way you did. They'll never get the unspoken nuances and tidbits that you know in your head, but just can't seem to communicate in words. I've found this happening when characters are behaving not quite the way they are supposed to be in my head. Giving materiality to the characters changes them. If you can get exactly what was in your head onto the paper, you can consider yourself a talented writer. I'm still working on that.