This article by the Telegraph's art critic Martin Gayford is interesting on the experience of sitting for Lucian Freud:
Freud's method is to leave areas of untouched surface visible until very late on. This is, as he characteristically put it, "because it makes it more difficult, which helps me". "Many painters," he explained further, "may like to feel that they are working on a picture that is already there. But I like to feel that every aspect of it is provisional, changeable, removable." Consequently, my portrait grew outwards from a single point - "organically" as Freud puts it.
More how we work.