It is not straightforward; there can be no formula. Each picture is approached warily, as a new problem. Some I do indeed fill from edge to edge; others are almost minimalist paintings. I have come to a kind of manifesto for myself — create depth but not distance, so that even in a "filled in" painting, I seldom set out to create a landscape in the normal sense of the word. In the more open pictures, I think (or, more accurately, feel) carefully about the white spaces. They too have their designed shapes, and their correctness can be spoilt by the intrusion of a single twig or grass stalk.
Nothing in all this is rigid; eventually, there are no rules. I don't believe that one can arbitrarily impose style. The subject, and sometimes the purpose of the painting, should set the style. For example, when I planned the pictures in the book The Waterfowl of North America, I conceived them as eventually hanging on a wall, but they were also composed with the book in mind. Consequently, the size and shape of the page were an influence, as were the limitations of the printing process. One of my triptychs, measuring nine feet by five feet, makes quite different demands of composition, style, and technique.
One has to nurture an open-mindedness while composing, a naivete almost. With each painting, we are granted a new start in life. Sometimes this new start is bungled. Studies are mad, rough colour sketches assayed. One attempts to put it all together in a felicitous way, but it sours. Occasionally I will be halfway through a painting before the dark, suppressed doubt surfaces and with it the realization that the picture is beyond saving.