Got an email from my editor, who's finished a line-edit of the first 300 pp. of the ms. She was disturbed that an important bit of information didn't appear until the 7th chapter, wanted to know if there was some way it could come in sooner. I thought about it, sent her a suggested place for it, she liked it, I wrote a two-paragraph insert for a scene, sent it to her this morning. Why didn't she notice this earlier? It's a forest-for-the-trees thing. I think it's a good sign: it means the book was reading smoothly to her up to then, and this one thing, small enough not to be noticed before, stopped her now. Which means she's probably comfortable with my responses to her editorial suggestions.

The technical way you deal with this, once the ms. has been submitted, is to write the insert (assuming it's more than a line or two) on a fresh page, calling it "Insert for p. 31," and indicating exactly where it goes: "Following line 14, add two new paragraphs." I also give the last 3 words before it and the first 3 after it, but I'm a belt-and-suspenders type.