Recently, I've been reading through the Fiend Folio because it ended up on my desk. Sometimes the D&D books just seem to float around, ending up places in a sort of Brownian motion. It's an interesting phenomenon I can't entirely explain. That aside, however, as I said, I've been browsing the Fiend Folio. In general, it's not my favorite of the monster books, although I like most of the art quite a bit. It has a gritty, dark feel to it that's not so much the folkloric feel of the Monster Manual or the somewhat clinical Monster Manual II. Nevertheless, many of the monsters are a bit silly or a bit contrived. Reading through them, though, I realized that several little adventures or encounters kept popping into my head. And I stopped and thought about this for a second.
I've said several times that my adventures tend to begin with a visual image, and that's true. Yet there might be a bit more to it. It may be that reading monster descriptions has a tendency to create those visual images as they derive from a monster description. When I visualize a monster based on its description, I often get a glimpse of the background behind it, summoned forth from my subconscious ... and since I wasn't thinking about the background when the monster picture got created, the backgrounds can sometimes be quite unusual, the same way that dreams create unusual settings out of nowhere.
It might be that the best adventures don't necessarily come purely from a visual image, but from the combination of a visual image plus some less-clear addition to the visual image that crops up somewhere in the mentally-supplied background. A picture of Angkor Wat might have the subsidiary, dreamlike impression that "something ripples that water," or "something moves in the shadow of that doorway." And then the interpretive part of the brain starts to fill that in, working its way from the vague into the specific.
The mind works in bizarre ways, sometimes. I've never been able to really pin down the mental process of coming up with ideas, and heck, maybe it happens in several different ways at different times. But perusing a monster book seems to be a really good way of letting background visual images crop up out of nowhere.