Defining Character Moments, Part II: Revealing Character In-Story

Actions speak louder than words, and a critical aspect of revealing your characters to your audience is through character action.
Each of us react to situations based on a combination of our inherent personality traits (yes, a big part of personality is biological!) and a lifetime of accumulated knowledge. Any response to conflict reveals volumes about someone's personality, worldview, and past experience-- or, later in the story, demonstrate important character growth.
As I've mentioned before (usually while ranting about Walter Scott), it's important to make sure that the traits displayed by the character's actions are consistent with their stated nature, and that the other characters have logically explainable actions to their behaviour. (I should note this doesn't mean a rational response-- but one that makes sense in context. For example, a character feeling anger because her BFF is moving away for university isn't rational, but if said character has established abandonment issues, it makes sense in the context of the story).
It's necessary that the reactions to conflict are unique to the character-- forget about what a generic person 'should' feel in a given situation, and think about how that character responds as an individual. This not only lends authenticity and makes your character more memorable, but allows for some surprising plot progressions as the characters don't take the 'expected' course of action.