While previous research on interpersonal decisions has focused on individual differences or situational determinants, in this paper, we looked at the interplay of situation and personality in decisions that are characterized by a conflict between self-interest and joint welfare. In an online experiment, 185 participants made decisions about the allocation of their own work time and the work time of another anonymous participant. Agency and uncertainty were manipulated between subjects, and social value orientation as well as dispositional envy were assessed. Participants chose between an option that maximized participants' joint welfare and an option that maximized personal payoffs. The results pointed to an interaction between personality and situational forces: Uncertainty moderated the effect of social value orientation, and agency moderated the effect of envy on decisions. Taken together, the results provide evidence that the effect of individual differences in interpersonal decisions depends on the situation. Implications for team work are discussed in situations where a group potentially benefits disproportionally more from an individual's relatively higher effort than the individual who exerts the effort.