You know how butterflies can't seem to fly in a straight line and never go where you expect them to go? It's really hard to capture them, even with a huge net. I don't even try. I just watch them fly and appreciate their beauty and attempt to imprint it on my memory.
Butterflies are like story ideas.
They wander into view unexpectedly, a flash of distracting gorgeousness exactly when I don't expect them and, frankly, don't have time for them. (Usually when I'm driving, exercising, or doing dishes.) The best I can do is attempt to see every part of them, run them through the challenge course of my brain, and attempt to imprint their essence there so I can retrieve them later, when I have the time.
Okay, yes, I have tried fishtailing onto a side street, skidding to a stop, grabbing my handy-dandy notebook, and furiously writing the thing down. Usually it's a dialogue snippet of such startling brilliance I find myself amazed... until I read it back later and am like, what? I almost got myself rear-ended for this crud?
Same thing with dreams: I'll wake up, certain I've got a complete and glorious story or scene ready-made from dreamland, and I'll scribble it down in a rush, only to find out later that it wasn't so great and actually was probably just a dream-mangled episode of Doctor Who or quest from Dragon Age.
Mostly I find that these brilliant butterfly ideas are only beautiful in the moment. If I write them exactly as they are, it's like capturing a critter in a net, and folks, that's not where a butterfly is supposed to be. A butterfly, like an idea, is only actually beautiful if it's wild.
So I started making myself step back and letting my ideas fly, and turns out they don't always fly away. Sometimes the linger, thread themselves in and out of whatever other task I'm doing, and then later, when I sit down to write, I find that all that aimless flitting has evolved into a discernible pattern and has sort of magically fitted itself into my work-in-progress. I guess, my brain being what it is, it's not the act of recognizing an idea that's useful: it's allowing that idea to process.
So maybe the idea is more caterpillar than butterfly, honestly. It's better if it has time to develop.