Part 1: Gereral Overview

Planning

Before you begin modeling your character's head you need to establish what it will be used for. Because we are modeling a head we can pretty much rest assured that our character will be used for something like a close-up or ECU (extreme close-up) shot. As a result, we can rule out a Wide Shot (or full body shot).
The images depicted here demonstrate the same character within a wide shot, followed by a close-up and finally an ECU.

The Shot Type

Certain information can be sourced from various departments or individuals involved with the project such as,
What kind of shot our character is involved in?
The of lighting that could possibly be used in the scene?
The format of the final render output.

All of this information and more should be acquired and taken into consideration when planning how to appraoch modelling the subject.

Regarding the shot type, we don't want to spend hours modeling a character's lower body when it won't appear in the final shot, so we'll focus on the most visible parts of our character, then thin our workload out towards the less visible areas.
This will also have the side-effect of creating a more efficient, workable model that hasn't exceeded our alocated timeframe for development yet meets all the necassary requirements for the shot type.