My eye is always drawn to those little places in a picture where the artist takes the liberty to play with abstract design. Sometimes you'll find artists indulging themselves when they depict folds, or water. Often you find them sneaking it in when they portray hair.
In this Joe De Mers picture (which I borrowed from Leif Peng's excellent Today's Inspiration blog) contrast De Mers' tight, disciplined treatment of the face and hands with his wild treatment of the hair.
The hair in this "realistic" picture is as abstract as any Jackson Pollock painting. It enlivens the whole picture.
Similarly, in the following wonderful illustration, Bob Peak has carefully constructed a picture with many intricate figures, but when it comes to the hair, Peak returns to the freedom of kindergarten fingerpainting.
This must have been fun to do.
Frank Frazetta is another illustrator who created realistic, highly detailed pictures but when it came to hair, he stopped worrying about the rules of anatomy or perspective or shading. He completely unleashed himself and let design have free reign.
Somehow, all of his figures seemed to be standing in small cyclones.
Frazetta's wildly flowing hair not only added important vitality but also served a major compositional purpose.
Robert Frost once wrote: "The moments of freedom, they cannot be given to you. You have to take them." Artists employed to create pictures have to satisfy many masters: art directors, clients, audiences, printers-- even the subject matter imposes its own compromises. The artist is not free to fling paint or blend colors solely to create primal beauty. Yet, working within all these constraints, the artist can usually find little moments of giddiness in the hair, or the folds, or the clouds.
Obligatory art-is-like-life sermon for those younger readers who might not have figured out this part yet: everyone operates every day under lots of constraints, but if you are good at what you do, and you care enough, you can seize back what Frost calls those "moments of freedom" and, like De Mers, Peak and Frazetta, make them meaningful to your overall picture.