The small area of optical acuity (3% of the central field) of the human eye means that our eyes need to flick from place to place, taking in visual snap-shots (saccades) which our brain composites into its interpretation of the whole. The process of looking - the pathway traced, and the duration of each saccade - comes in two tasty flavours: spontaneous and questing.
Spontaneous (i.e. when we're just taking it all in)
In this mode our scan-pattern is light and 'flaky' - flitting to points of novelty, sophistication and congruity. Visual weight is an important determinant, where the heaviness of attractants is modulated by factors such as the instinctual e.g. eyes and lips; and cultural e.g. colour, geometry and symbols.
Questing ( i.e. when we're actively looking for a piece of information)
When we are looking for one or more particular pieces of information, the way we look changes. Hence our state of mind, in this case the expectation of finding something, influences the way we look even before we begin the process. We weight visual elements informationally, according to how important we think they are as clues to what we expect to find, consciously overriding visual weight.
Creators in the visual arts believe they can direct the viewer's eye; 'Intended Order' is a founding premise in visual composition. However although there is broad agreement amongst viewers as to which parts of a composition carry information, the interpretation and weighting of that information is modulated by each viewer's life experiences.
And for compositors, Michael Freemen makes an interesting comment that "most people decide quite quickly what they think is important and/or interesting in an image, and go on looking at those parts" (in 'The Photographer's Eye' page 60). In other words, viewers re-scan the same informational points instead of looking at new parts.