What's his attitude? How to react to situations?

The face study is a vital part in animation, i'm here mentioning only the importance of each steps and not the step by step procedure as it will be presented later. Try to draw maximum face reactions that your character will make and how he / she reacts to any given situation.

Expressions can be achieved by moving eyes , eye brows , mouth mostly. Each part of a face should be handled with key interest and importance so as to achieve a natural act.

for example if the situation is like " everybody likes me " , then the eyes will be bigger and eyebrows high up and mouth with a smile. these concepts are an easier to visualise or can be studied from a real persion.

Animating the facial expresssions giving finer details will make the animation really interesting! .

Dont forget the bacics exaggeration, anticipation, follow through etc.