I get requests for tutorials on how to translate real people into more stylized but recognizable versions fairly often, so here is a very wordy/slapped together tutorial for how I study faces! Specifically for how I go about breaking up the face into simple shapes and ratios that are easy to carry over into different styles.
I used Chris Pine because I haven’t drawn him before, and I figured it would make the process more genuine.
EDIT: So this has been up for ages and I didn't think it needed to be said, but apparently some people are confused. This isn't saying "just trace a photo"! The tracings here are meant to show how I break the face up into different shapes. Knowing how to see the face as shapes makes it a lot more accessible and easier to portray in different styles. If you simply trace a photo instead of learning how to draw it on your own, you'll have a very hard time injecting your style and energy into it, and the end result will probably be very stiff and lack expression.
Here's a few other examples of how learning to identify shapes and relationships can help you translate faces into different styles, ranging from semi-realistic to way more stylized. And don't be afraid to stress the unique qualities and variety in individual faces!
The Hobbit dwarves: [link]
Charles and Erik from X-Men First Class as X-Men Evo cartoon characters: [link]
Some Thor studies eyeballed from references: [link]
Supernatural sketches done while watching an episode: [link]