From my own experience back in college, an eyewitness can be spot on some details while completely blurry on others.
I was crossing a street near the edge of my college campus in broad daylight when a car slowed down a few feet away from me. The male driver had his window down and asked me for directions to McDonalds. I walked forward until I was about a foot away from him, kind of looking up at the sky as I tried to formulate directions. When I looked back down, I saw into the car and he was exposing himself and furiously self-pleasuring. I screamed, backed off, he started to speed away and I began to run to the nearest building. The whole encounter lasted.....30-45 seconds?
But anger flooded me--I stopped and stared at his license plate and memorized it until I couldn't see it any more. I chanted it to myself while I ran into the building across the street, ran to my boyfriend's office, and wrote it down. Called campus police right away.
What did I have clear visual memory of at that moment? The license plate number and state. The color of the car. His smirk and shape of his nose. His genitalia (please no line-up of that!) and his hand.
Less detailed memories: general kind of car (I was not a car person...best I could get was 4 door sedan...no make or model). Specific hair color--just dark, either brown or lighter black. Dark eyes. General shape and expression of face. Distinct feeling that he bore a resemblance to Pee Wee Herman's mug shot when he was arrested in an adult movie theater (that had happened just about 2 months earlier and the photo had been in magazines and tv news). Sort of age--late 20s-30s? As a 19 year old, all I knew was he was older than me, but not "old".
Things I couldn't remember at all: his voice, other than nondescript (I remembered his exact words, though). Any details of the car like make, model, dents, bumper stickers. What he was wearing. Specifics of his hairstyle,
I gave the campus police as much info as I could. They were so excited to hear I had the plate number and told me that this guy had done it at least 5 other times on campus--but I was the first to get the plates. I tried for an hour to come up with any other specific details. I tried to describe his clothes without a lot of success.
They called me back that night and had me come look at a photo line up. My heart sank when I looked at it, because all I could see in my head at that moment was Pee Wee Herman, which my brain had started to superimpose over his features in my memory because of the resemblance. I did start to study one picture more, but I couldn't be sure. The officer noticed that and said that it might help me if I used my hand to cover up the hair on each photo and just look at the face.
And that's when I recognized him. The vague resemblance to Pee Wee Herman's mug shot was there, but not as strong as my mind had started to insist it was. I realized also that my brain had been putting him in a white t-shirt like Pee Wee's mug shot--but that it had actually been a dark color that contrasted with his arm.
It was him--the license plate matched to his girlfriend's car.
I don't know that I would have been able to help a sketch artist get a recognizable drawing of him, if I hadn't got the plates or been given a photo lineup. It would have been close-ish, but not great. And the one thing my mind had an absolute crystal clear memory of definitely wouldn't have been put on a suspect drawing.
When I saw him at court, he of course looked completely different. Different hair cut, got rid of the goatee, suit, etc. He even had a pair of glasses he kept putting on.
And I got put through the ringer by the defense when I was on the stand--their first question was what was I wearing when it happened (ah, the good old days of 1992..I was wearing knee length cut off jean shorts and an oversized tshirt). What was I even doing in that location at that time of day (um, going to the computer lab on campus where my boyfriend worked at 5 pm.). How did I know that it was his actual male organ that I saw and not his thumb--I can't even remember how I managed to answer that in a non snarky way because I was dumbfounded by the nerve of the defense attorney. Why hadn't I noticed and reported that his client was shaking like crazy (because he had told police he had taken 1/2 a bottle of caffeine pills that day)--um, because he wasn't and that was a stupid lie he told.