I love painting hair, even when it is not hair....
One of my heroes, Ivan Albright (did you notice that the Jim Broadbent character/crazy artist in the dark apartment in Art School Confidential had an Ivan Albright reproduction on his bulletin board?) said that when he was painting this painting (a detail of "That Which I Should Hve Done I Did Not Do"), he "was thinking of Jesus when he painted one rose, and it was by far the sweetest of them all".
I have been reading Twyla Tharp's The Creative Habit (one of the best books I have read in the recent past), and she calls metaphor "The lifeblood of all art". She also writes a great chapter on what she calls "spine", about your internal inspiration for a piece, which is often not evident to a viewer looking at the finished work, and would be distracting to a viewer if you even mentioned it (better to keep it to yourself), but vital to the creation of the work.
I was doing a visiting artist stint at University of Alabama and doing a crit, when I was finally able to articulate what a certain student needed to do, and afterwards thought, "Hmmmm... I think I just gave away the store...". I think that there are lots of artists who are channeling or summoning something up, whether consciously or unconsciously, and holding it in their minds as they work, so that there is always at least one more intangible layer in the work, sometimes more. Like thinking about the way your father's veins stood out in his forehead when he yelled, and painting drops of rain running down a window. Thinking one thing, and painting another.