My dad has a fierce and unruly temper. I was shocked when my sister-in-law said she had never seen him angry, because I always saw him angry at least a few times every day. It could be from watching a football game he didn't care about, driving, someone interrupting or contradicting him, or dozens of other little things. He's angry all the time... except when entertaining company. He can control his temper; he just doesn't do it around family.
All people are that way. Our behavior and even thinking changes drastically from one social group to another. Different situations call upon different aspects of our personality, and cause us to emphasize one more than another. Each person has only one personality, but each personality contains many faces.
Sadly, game characters aren't that way. For all the progress that's been made in game stories, we're still seeing characters with just one face to show through the entire story. Game writers are still avoiding depthful personalities.
A depthful character isn't merely brave. He is brave in some situations and not others. The man who would readily risk his own life on the battlefield might be terrified of public speaking or mingling at a party. The socialite who effortlessly mixes with strangers might be less sure of herself when facing persons with whom she has deep relationships, like family. The warrior who could wrestle a lion might be afraid of the water, unable to swim.
A depthful character isn't merely loving. What one person considers tough love, another consider cruel. What one person considers acceptance, another considers willingness to condone the other's self-destruction. A depthful character has a specific and personal conception of what love is.
Don't try to offer depth like this through dialogue. Dialogue limits the player to the developers' own preferences and foresight, as Mass Effect did. The player's actions are certainly limited as well, but the fuzzier definition allows the player's imagination to reforge events closer to his or her true intentions. Generally, it not as important that the game know why I choose a particular action as it is that I know why. The story of any game does not exist wholly on the screen.