To really make your character look alive, you're going to have to give it an expressive face. Anger, joy...there's no shortage of emotions!
Have fun playing around with different pairs of eyes and a variety of mouths. You'll quickly realise the myriad possible combinations at your disposal...
Here are some tips for bringing out your character's feelings
The rest of the expression must of course be logical and coherent. Practice moving gradually from one emotion to another: for example, your character can go from having a doubtful expression to one of amazement and finally one of outright surprise!
Print or use this image as inspiration to complete the missing expressions with your own personal style.
A little anecdote: Bill Watterson, the cartoonist of the extremely expressive Calvin et Hobbes, couldn't believe he was getting paid to spend his days making faces in front of the bathroom mirror to draw Calvin.