There are times when even a good artist upon have to draw faces and people get nervous or worried. The reason could be varying. They might think of the consequences of getting the proportions wrong. What if my sketching depicts Mr. Jackson to be 80 lbs bulkier than he actually is? What incase the figure seems stooped? What is the painting I am making is nowhere near the reality I'm attempting to sketch? These fear constantly worry an artist, irrespective of the number of year of experience behind that artist. At times capturing a person isn't as difficult as capturing their likeness is.
Similarity is of utmost importance when sketching people. There is a principle that is attached to seeing people correctly and organizing of images too is highly important while capturing a similarity of that person you intend to draw. Take for instance, sketching the eye. Many a times artist sketch the eyes of the figure while attempting to envision 2 footballs. It might be either the artist relates to eyes as being similar to a football or he has been taught like this and he does it always. You have to shake off such icons and only then would you be able to draw a more similar eye of the person.
For an artist to sketch a generic person who is like no one is fine but one who can envision the core of the subject at question and lend it a new life is face drawing. What you related to as icons as a youngest aren't relevant now and seeing the subject accurately is relevant to draw precisely.
Learning drawing is as good as learning a fresh language for which forgetting some of the older basic is necessary to be able to adapt quickly and have a fresh perspective of the new subject. Getting to know that the eyes can have different variations & shapes and that every eye is matchless shows that our perceptive of them having a football like appearance isn't correct.
When a figure is posing, it is the artist who processes this info and then draws, the drawing isn't what he notices but is what he assumes he notices. For the same reason drawing is viewing precisely. The key is to give more attention to the things that you see, and thereafter the outcome of your drawings will be more precise and you shall move in the direction of achieving closer similarity of the subject.
Drawing is an art that anyone can learn. It begins by having practice, determination & passion. When you start learning the correct methods of doing studies and observing, implementing these techniques shall see a huge improvement and witness growth as an artist.