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Michael G. is correct that graphite (due to the "platelet" nature of its structure) can eventually "strike" through paint layers, becoming visible at the surface. It's possible an isolating layer of "fixative" might stop that from happening, but it would be better to use drawing materials known not to strike . . . e.g. charcoal (vine or compressed), Nupastel, Stabilo pencil, all "stay put" when overpainted with oils.
It's problematic to refer to "fixative" as if it were always a specific material of known composition and properties. Most commonly marketed aerosols are nitrocellulose lacquer. As you might expect, there's a fairly wide range of formulae for pastel or charcoal fixatives, almost all are "spirit" (alcohol) soluble, which qualifies them as an "isolation layer" when used with oil paints. Most are insoluble in oil, turps or MS.
I believe Allan is correct to say the overpainting will penetrate a drawing under fixative, since in this case, it's unnecesary (and inadvisable) to apply fixative so heavily as to isolate the ground entirely.
Using fixative over a preliminary drawing is a matter of personal preference. Sanden may wish to be able to remove initial paint applications without disturbing the drawing, or he may simply object to the pencil marks "lifting" enough to discolor his paint, or perhaps having tested his materials and methods, he is satisfied graphite will not strike by doing so.
There may be an even greater number of respected painters who start to paint directly on a ground than those who begin over a detailed drawing.
Abbey, Parrish, Rockwell, and numerous other painter/illustrators of their era commonly used shellac as an isolation layer over preliminary drawings and between paint applications. Some of their paintings are in pretty bad shape, at a relatively "young" age, which might suggest that the use of isolating layers was the cause . . . or not?
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