I remember studying this. They compared a group of people who they told to fake a smile and a group of people they told to fake a frown. The smilers reported being happier than the control group and the frowners reported being sadder than the control group. I forgot what they did, maybe watched a video or looked at pictures.
No, you can fake a smile to show people that your not sad, and that nothing is wrong. But when in reality theirs a bunch of depression going on in your life.
Some people show a smile just so people don't ask you "Hey whats wrong?", and then you end up crying.