Maybe it’s just me, but I don’t think I would be able to describe even someone I knew well in such detail that an artist could make a photographic quality drawing of them, let alone somebody I saw for only a few seconds in the midst of them committing a crime. I might be able to point out a few obvious features, like if the guy had a beard, a big scar or a nose ring. But if it’s a fairly ordinary looking guy I would have no idea where to start.
So how do police sketch artists manage to get such good drawings of suspects? Are there a lot of cases where a person isn’t able to describe to them in any real detail what a suspect looks like?
I don’t know that much about it, never seen it done, but I believe that a large part is the process of drawing something fairly generic or average, and by comparison the witness can usually point out something that’s wrong with that face and how it should be changed to be closer - successive refinement in terms of facial details.
“Okay, so the eyebrows have to be bushier - bushier - bushier - well, not quite that bushy - yeah, that’s right.” 
That’s what I’ve assumed.
I know nothing about the process, but I would assume chrisk has the right idea. Five years of working at a print shop has taught me one major life lesson : Most people don’t know what they want, but they know what they don’t want. If I witnessed a crime, I may only be able to tell the cop "I don’t know. He was a purple guy with green hair and a skinny face (which may seem easy to spot in a crowd, but let’s not profile). But given a baseline to start with, I will suddenly remember that his ears were larger than average and he had wrinkles on his forehead. This may trigger even more memories, such as the top of a tattoo poking showing above his t-shirt.
There is also a great deal of variation in how well people remember faces in particular. I could never understand how people could provide details about ears and eyebrows, things I couldn’t describe for anyone but my closest friends and family members. Then I started reading about face blindness (a biological inability to recognize faces), and also about how people from different ethnic backgrounds identify people (using different characteristics based on what differences are most apparent in their ethnic group), and I came to the conclusion that I fall on the very low end of the scale of recognition. I’m not exactly face blind, but I have to know a person quite well and have seen him or her a relatively large number of times before I have any sense at all of what he or she looks like. The many, many times I have called someone by the wrong name, or made an introduction to someone only to learn that I’d met them before, or had someone recognize me while I had absolutely no idea who they were all began to make sense to me.
Now I just assume that others have a far more highly-developed and detailed memory for faces that I do, and hope that I’m never called to be an eyewitness.
During the interrogation of Susan Smith (the lady from SC who murdered her two kids) police first became suspicious of her when the was describing the “assailants”. She went into extreme detail on everything, and the cop in charge said that that type of thing never happens. Usually someone knows one key element of the face and everything else is muddled together…