First of all, what does developmental editing mean? Another term is structural editing, or concept editing. Developmental editing addresses the large-scale issues of the manuscript, such as character and plot development all the way through the book. It is the macro phase of the edit in relation to the line edit, which is the micro phase.
The most important job for a fiction editor is helping authors develop their characters and their plots. I provide specific suggestions right on the page to point out ways in which the novel might be improved. These notes fall in three basic categories. I’ll write a note if I feel that any new material might be added, from a character’s thought to an entirely new chapter. Other notes are written if I think material might best be cut, usually for pacing. Sometimes a section or scene might be more effective if moved elsewhere. By the end of a manuscript I will provide 15–20 pages of suggestions that lay out an entire developmental road map that can be followed, if the author wishes.