Research has demonstrated high levels of consensus and self-other agreement for extraversion and conscientiousness. However, the mechanisms whereby these assessments contribute to accuracy in behavioral predictions remain unclear. In this study, two judges rated targets on Big Five personality factors, and predicted their compliance to offer help in response to a fabricated emergency. Whereas one judge interacted directly with the target, the other judge only observed the interaction. There was consensus and self-other agreement on conscientiousness for both judges. Critically, self-reported conscientiousness fully mediated the effect of judged conscientiousness on compliance to offer help, regardless of the nature of the judge–target interaction. These results demonstrate a mechanism whereby judged personality can predict a specific behavioral outcome in initially unacquainted persons.