@stuartk, @mahgister
I think we might be talking about the difference between idea/concept and action, and I think that relates to music. I think many, if not all, musicians find their music in action first and then record their ideas. A plain example would be Mozart sitting at the piano picking out notes in a certain order, shaping time.
No doubt composers also compose in their heads, but I don't think they're seeing the notes written on the page. I think they are hearing the notes in their heads and perhaps then playing the notes on an instrument and then recording them. If that is the case, then the idea/ concept follows the action of hearing the notes and the timing.
Malcolm Gladwell wrote a very interesting article on art called "The Late Bloomers." In case anyone wants to read, here is the URL
https://www.newyorker.com/magazine/2008/10/20/late-bloomers-malcolm-gladwell
His basic theory is that artists either compose a work in their heads or they find it on the page, so to speak. Two examples he used was Picasso and Cezanne. Picasso, he said, was able to see his work in his head and then paint it. (Although I wonder, because I have seen Picasso studies for Guernica.) Cezanne found his painting on the canvas. Jazz musicians find their improvisations before they can think them. They have to bypass thinking, which I believe makes the best art.
I worked on a Mark Rothko-inspired painting for months. I could not find shapes on the page that came alive. It must have been at least five months before I woke up one morning and said to myself, "That's easy." I had the painting solved in days because I felt it rather than thinking about it.
On a more mundane level, I heard that Kobe Bryant once told Pau Gasol, "You're the best center in the world. Stop thinking about it and just shoot." Here action doesn't just precede thought, it bypasses thought.
I think the greatest musicians are able to bypass thought and pluck the notes out of the ether, Perhaps what they call God. As Milton said, "I did not write Paradise Lost. God wrote it through me."