Size is certainly the most crucial of the four factors. After a great deal of observation, there is no question that these fish subject small insects to much less scrutiny. I regret that I didn't time such examinations with a stop watch and construct charts demonstrating the correlation between duration and size that I know to exist. Maybe I'll get around to it next season . . . Movement is only slightly less important than size.
Inspection is longer and more intense if the insect is either completely dead or extremely active.
Motionless naturals commonly drift unmolested for several minutes, and are then consumed mainly by juveniles or, sometimes, are never taken at all. Big bluegill show great interest in a violently thrashing insect, but will rarely mouth it until the struggling subsides. They will mill around a moth, for example, that is beating it's wings, apparently waiting for it to settle down. Subtle, accurate animation is, therefore, the name of this game. Suspicion is further aroused if the natural is unusual.