I have this peculiarity about my creative projects; I tend to come up with soundtracks for them.
It started when I was in junior high school; I'd listen to music and think about my stories, and this or that bit of music would inspire an idea or two; and pretty soon the music I was listening to became the soundtrack of the stories I was writing.
When I got the ability to make CDs, though, the whole thing took on a new dimension.
I started with a project that currently has the working title of Renaissance Man; I took the conditions of Larry Niven's For a Foggy Night and extrapolated what it would be like to find yourself in a parallel universe.
I had recently digitized some music by Jim Lillquist (Ernie's Journey was the album title) and realized that the music fit my story perfectly.
I didn't have the song names--I had a copy of a cassette--so I just made up new ones (which fit my story) and saved 'em. And so the story felt more real to me, more immediate, and I was able to write some good stuff while listening to music that came from that world.
...and later, with my manga efforts, it continued. I came up with an OST for American Dawn after deciding that an image song for Magical Girl Pretty Sammy would be the perfect OP for AD ("Magic Door", in case you're wondering and happen to have the OST disk laying around).
So here I am now, listening to the OST for Chicory on my MP3 player.
The OST for Chicory started with Liz Story's "Snowfall" from Winter's Solstice VI. I had the Christmas music channel on last November, happened to hear it, and liked it; and after Chicory started gelling in my brain, "Snowfall" was the obvious OP for it because the story opens with Subaru walking home from school in a snowstorm. When I play the song, I can see what the OP sequence for the first episode would look like.
Anyway, a couple weeks ago I found a whole bunch of John Jarvis in my music directory, so I dumped it to the MP3 player. I hadn't listened to JJ's music in quite a while, but in 1990 or so I was really into it; the songs were familiar and relaxing.
...and as I listened to his song "A Month of Seasons", suddenly I had this vivid mental image of Subaru skiing with her friends. I mean, if there was just some way to transfer what was in my brain to video, you could be watching Chicory anime right now instead of having to put up with my droning. The scene was complete with music, sound effects, voices, everything.
There are a few other songs which evoke images for the story: Stanton Lanier's "Awaken the Dawn", for example. All of the music is keyboard music, and it fits as if precision-machined.
Don't ask me why. I wish I could understand my creative process, and harness it for good (or for awesome) but unfortunately I don't. I just know that when it comes to manga, I can't make it happen and I can't rush it...and frequently I can't control what comes out, either. I have talked before about The Hand, how it just rearranges my manga stories to suit its own nefarious plans; but it is a better artist than I am, and seems to have a better grasp (heh, "grasp") of the fundamentals of comic art than I do. It just does things and I look at the result and have to say, "Well, this is better than what I had planned...."
My writing is much more tightly controlled; that flows right from the brain to the screen, almost as if by magic. I still don't know how my ideas form, where they come from; but at least I am familiar enough with the process that it rarely startles me the way drawing does.
(It really isn't a bad thing--I joke around about it being sinister, but really it's just how it works with me. It's not scary at all.)
So when there is music that fits a story, I'll use it: anything to help the creative process work a bit more smoothly is welcome. (Well, as long as it's legal and non-addictive.)