I have overheard fine artist and former illustrator Rick Piloco dispatch the question by framing it in terms of intentionality: "illustration is just painting what an art director wants me to paint, instead of what I want to paint." Piloco has a gift for no-nonsense incisiveness, but I propose that there are certain stylistic hallmarks that mark the difference.
What does a "shtick" look like in practice? Just like the difference between pornography versus an Ingres odalisque, I know it when I see it, and what I'm seeing is the use of symbolic visual shorthand instead of an authentic visual experience. Malcom Liepke's work presents a good case study since it is formidable and not a "straw man" that is easy to knock over as "illustrated." Liepke is considered to be a leading figurative realist, so his work is a valid target for scrutiny. Looking at Malcom Liepke's older restaurant scenes from around 2003, all the figures look the same. The noses on the faces are the clones of the same bulbous shape and exaggerated plummy red. Does Liepke only hang out with people who have colds? More likely, he verifies its orientation and placement among the larger shapes of the face, and drops in his standard nose in an artful copy and paste.
(Above: a delightful scene by Malcom Liepke, with an "illustrated" feel.)
Don't get me wrong, Liepke's narrative figure scenes are delightfully cinematic, playful, with inventive color schemes of complimentary reds and greens. The paint quality is luscious and fresh. But, alas, in my view his work is the essence of "illustrated" fine art. Of course, greenish backgrounds, scarlet lips and puffy roascea is his stylistic "signature" which makes his work recognizable, but to Liepke's well publicized dismay, his visual shorthand is readily copyable. Indeed, a host of second-hand Liepkes have been popping up everywhere (especially in the pages of American Art Collector), each imitator hoping to siphon off his commercial success. Unfortunately for Liepke the copies look an awful lot like the real thing.
(Self Portrait by Kate Lehman, 2003. A stunning fine art counter-example to an "illustrated" approach.)
This optical naivete, informed, but not overruled, by intellectual understanding is also evident in the quiet emotional power and of portraits by Travis Schlaht and Kate Lehman. The border between illustration and fine art is a hazy one, but the edges of the split hair span a vast chasm between Sargent's subtle anatomical shorthand and the formulaic recipes of many commercially successful contemporary realists.