Although results from cross-sectional between-person studies suggest a differentiation of employees in good and bad performers, recent studies have challenged this assumption by showing that performance is also dependent on more transient states that vary within individuals. Acknowledging that individuals do not only differ in reference to others, but also in reference to themselves, we integrated the between- and within-person approach in the examination of organizational citizenship behavior (OCB) and counterproductive work behavior (CWB). We propose a model informed by conservation of resources theory in which OCB and minor CWB are predicted by within-person variation in one’s level of vitality, with these relationships being moderated by trait core self-evaluations (CSE). Moderated multilevel Poisson regression analyses revealed that vitality was positively related with OCB and negatively with minor CWB. CSE moderated the relationship between vitality and OCB so that individuals high in CSE engaged in OCB regardless of their vitality levels; however, contrary to our expectations, CSE did not moderate the relationship between vitality and minor CWB. Together, these findings indicate a complex reality underlying the mechanisms that drive the enactment of OCB and CWB.