What makes your characters emotional in the first place? What/who do they really care about? Who would they sacrifice themselves for and why? If emotions do get the better of a character that normally remains bottled up, how does this affect them afterwards? Are they determined not to "let it all out" ever again or have they found this liberating?
Think about what causes your characters would care about and be prepared to act on. Think about their beliefs and how they would react if they were challenged on them. How do your characters handle ridicule and insults? Do your characters react the way they do because it was the way they were brought up or are they going in a different direction, deliberately, from their family?
Your characters' emotions can be shallow or deep but there should be good reasons behind them that a reader can identify with, even if they don't sympathise.