During selection procedures, obtaining reliable and valid estimates of the extent to which applicants are honest, emotional, extravert, agreeable, conscientious, and open to experience is of paramount importance for organizations to create an effective and committed workforce. In our research, we focus on the six-dimensional HEXACO model of personality, which—according to cross-cultural lexical research—offers a more optimal description of personality than the more well-known Big Five personality model. By using a Situation-Trait-Outcome Activation (STOA) model, we investigate what effect personality has on, in, and through work-related situations. When investigating personality, we especially focus on what we call the Three Nightmare Traits (TNT): low honesty-humility, low conscientiousness, and low agreeableness, which can have an especial detrimental effect on organizations, especially if a TNT person is in a leadership position. Furthermore, we are interested in response biases (e.g., social desirability) in selection situations and how these can be reduced by using more unobtrusive measures of personality, such as games or through speech analysis. In the course of our research, we have constructed a number of personality, leadership, and communication styles instruments that are widely used by practitioners in the Netherlands and Belgium.