An unconscious defense mechanism characterized by behaving in a way opposite to one's actual feelings is

projection
denial
sublimation
reaction formation
introjection

reaction formation

Rationale: By acting opposite to your actual feelings, such as laughing when you want to cry, an individual forms a reaction to a situation. The other terms listed are defense mechanisms that operate in different ways such as adopting another's values for your own, blaming others for your feelings of inadequacies, channeling or suggesting other more socially acceptable behaviors, or refusing to accept a situation.