Fan art can be tricky. Our features - a quirkiness in a eyebrow, set of the mouth - define our personality. All this must be captured with the brush or pen. Something slightly off can shift the portrait from right on target to "well that sorta reminds me of that person.."
Proko is a good go to reference for the basic breakdown of the human face...and just about all figure drawing. Check out his video library and blogs.
On a personal note, Mr Scott is always a delight to draw. He was really the only sane one on that ship.