The Distinction between plot and structure

" [Stevenson] mapped out the plan of a coming novel when- ever he resolved starting on one. This was putting up the scaffolding, he said; and as soon as the foundation was laid and the walls begun, he called attention to the rising structure." E. Blantyre Simpson, The Robert Louis Stevenson Originals
PLOT marks out the rough lines upon which a story is to be built, while structure completes the specifications for the building. Upon the workman- ship of the one no less than of the other depend the beauty and the strength of a story. A single error in plotting may make the whole unstable and tottering; a single flaw in structure may ruin the symmetry and mar the grace of a story. There is an art in sketching outlines and another in arranging the minutia. Structure concerns itself with the latter. It involves the work of details, of weather- boarding, of chimneys, of window-frames and doors.' It is used, as a technical term, to denote not the finishing, the final careful polishing of a story, but rather the minute elaboration of the plot preparatory to the actual work of writing. It is the altogether necessary, if slightly irksome, process of perfecting a Design before one can begin to make it appear in haunting word and living phrase. Every detail must have a reason for its existence and for its position.. Each sentence, each incident, each character, each Description, each remark, must be chosen for its harmony with the single effect and fitted into the place where it will count for greatest strength.1 Parts must, be so joined that the sutures do not stand out boldly. The skilful story will seem complete only when every part is so necessary that, were it removed, there would be an evident lack. In the structure of the story there belongs the consideration of effective subsidiary incidents and characters, of proportion, of order of events, of verisimilitude, of point of view of the "angle of narration." 
In every story, there are likely to be some characters which are not necessary to the plot movement, but are necessary to structure. These, one calls developing characters, for they assist in the growth of plot into the complete story. Often they appear for contrast as emphasizing foils.