Here’s how it works – at least for me. I was walking along, thinking about a scene where Claire follows King Ambrosius across a very large room in near-darkness. In the second draft, I had described everything Claire could make out in the gloom. Being highly visual, I hadn’t thought about what Claire would likely notice under those conditions – sounds – or what she would learn from what she could hear. So I added something in the third draft:
The king had a rolling gait like a long-time rider of horses, and his footfalls on the right hitched a bit from some old wound.