I'm looking for a word, or concise phrase that describes something. (So, this isn't just a single-word-request, and I'm unsure how to tag it.)
First, some examples of things that are close, but may not quite be what I'm trying to describe:
Visual artists (painters, sculptors, etc), often get a vision of something, then try to render their vision concrete using a medium and/or particular style.
"When Michelangelo looked at the block of marble he was to carve he looked beyond the outside and saw the shape of the statue he was about to create."
Musicians often have a 'sound' (or melody) in their head, that they try to make real.
(I can't find the good quote I recall):
"You will never come on anything in nature that sounds like a symphony."
So, for those artists it's not unusual to say that what they're aiming for is fully-formed in their head (yes, I realize not all visual or sound (aural? auditory?) art is created this way). The only problem the artists with these visions encounter is the time, labor, and the struggle with materials and technique to get something close to their vision out into reality.
But it is unusual, but not unheard of, for authors to get something like this artistic vision. Sometimes the muse (or monkey on their back) is beating down the doors - and things blossom, fully-formed - just sitting in their head, just waiting to come out - they flow.
"Well, The Princess Bride opened itself to me. I never had a writing experience like it. ... and it all just came out."
"The first time I wrote a real play, I was in college, eating breakfast, and heard a single line of dialogue echoing over and over in my mind. Intrigued, I moved to the typewriter (yes, it was that long ago) and took dictation (there's no other word for it) for two straight hours during which the characters, their plight, and every aspect of the drama was thrown at me at blazing speed without a moment of hesitation, doubt, or contemplative thought. The play (a one-act) was produced in college, produced professionally, and hardly a line changed from that first astounding burst."
A word that doesn't quite capture this experience: Promethean (the experience or result doesn't have to be boldly creative, which Promethean implies).
@vstrong has mentioned, and I have thought of describing it as Athenian, but is that a recognized/popular meaning for that word (is that word popularly used, even)? If I use that word, are most literate people going to understand my meaning - or are they going to have to derive it from their knowledge of Greek/Roman myth (scanty in this day and age) and contextual clues? Because mostly, when I run across that word, I associate that with residents of Athens, Greece - and I have a fair grounding in myth, which it seems most people do not.