Your demeanor is defined as being 

either your facial appearance 
or your behavior. 

When playing poker, don't let your demeanor give away how good your cards are.

Demeanor in English today has evolved from Middle English and Old French to refer to the way one manages or presents oneself, and this definition applies to conduct as well as facial expressions. 

The way you present yourself often affects how others make judgments about you and therefore, you might want your demeanor to reflect the situation you're in. 

So, if you're at a birthday party, your demeanor should be happy and smiling.