Charlton Heston was once asked whether or not playing Moses in Cecil B. DeMille's The Ten Commandments had a significant influence on him outside of his acting career. His answer was that it invariably did. Although Heston was far from being anywhere close to the caliber of Moses (as would anyone else, for that matter), simply playing this biblical hero in a movie affected the way he behaved off the set.
Here are some famous scenes from that role, which are quite apropos for Passover:
The lesson we can take from Heston's experience is that acting a certain way can - and often will - influence a person's overall behavior. If we train ourselves to act happy, we will become happier over time; if we consistently act dejected, we will become miserable over time. We don't have to be Moses - we just have to be ourselves and in control of our emotions to the greatest extent possible. While it's only human for emotions to get in the way of more civilized behavior from time to time, we can almost always overcome those feelings by acting in a different manner. In other words, our behavior should dictate our feelings more than our feelings should dictate our behavior.
Here is one question that might provide some clarity: is it fair to be rude to others until our feelings are willing to cooperate? Of course not. In this light, there is a principle known as mitoch shelo lishma bah lishma - something initially done without sincerity can ultimately lead to sincere performance. For example, getting in the habit of smiling for other people - even when we don't particularly feel like it - can lead to smiling on a regular basis with more sincerity. As Andy Rooney once said, "If you smile when no one else is around, you really mean it."