Once you have an idea, it wouldn't help to sit down and keep brooding, brooding, brooding...just live on but keep it in the back of your mind all the time. Keep connecting little bits and pieces that belong to it. Sometimes it's only a word, sometimes half a line of dialogue, sometimes an image that you squiggle down. And when it kind of in this way materializes, then press yourself with urgency.
— Werner Herzog
From a Reddit AMA, found here.