If you’re new to copyediting, consider these snippets of advice. (Be sure to check with your supervisor if any of them contradict your usual instructions.)
Don’t query a word or spelling or locution without looking it up. If a writer uses an unfamiliar word or spelling more than once, it’s very possibly intended. It’s easy to paste “eat one’s cake and have it, too” into a search engine and learn that the writer doesn’t have it backward.
Don’t waste a writer’s time by continually asking for approval. (“Okay? If you don’t like this, I can put it back.”) Rather, indicate your flexibility in the cover letter. On the manuscript, use queries for giving or asking for information.
Save grief later by e-mailing the author before you make editing decisions that are hard to undo. (“Re romantic/Romantic: do you have a system for capping? Should I meddle?”)
Don’t track changes that will be invisible or confusing on a black-and-white printout, such as deletions of hyphens.* If the editing is difficult to read, the writer won’t easily see that the results read well.
Be conservative in editing until you have more experience. You should be ready to explain every mark you put on the page.
Remember the copyeditor’s creed: First, do no harm.