When writing about a character's traits and personality you want to give a well-rounded overview. In order to do so, it is important to look at a variety of aspects. A great method I teach my students is STEAL:
- SPEECH: What does the character say? How do they speak?
- THOUGHTS: What is revealed through the characters thoughts and feelings (usually established through first-person narration)?
- EFFECTS ON OTHERS: How do other characters feel, behave, or react to this character?
- ACTIONS: What does the character do? How does the character behave?
- LOOKS: What does the character look like? How does he or she dress?
The important thing to remember is that as you approach each of these points you should be careful to attribute meaning for each example. For instance, when we study Brave New World we use this method to characterize Bernard Marx. When Bernard tries to disassociate himself with the Savage and Hemholtz during the riot for soma, he is displaying cowardice. I really push my students to attribute a specific trait/adjective to the examples they cite.
Hope this helps. Also, I don't take any credit for this method; it was passed on to me from another teacher. A quick google search for "characterization STEAL" will turn up further information about this method.