Does a very brief observation of a person predict first impressions? Prior research has shown that these brief observations, called thin slices, predict many psychological outcomes such as individual performance. However, there is a not much research investigating whether thin slices predict first impressions formed following live interactions. In the present research, one hundred female participants were asked to complete three 15-minute face-to-face interactions in dyads. After each interaction session, their explicit and implicit warmth about their interaction partner was assessed. Ten observers rated these participants on warmth, competence and attractiveness based on brief silent video clips extracted from the interactions. Multilevel analyses revealed that for a given participant, observer-rated attractiveness (but not observer-rated warmth and competence) of their interaction partner predicted greater implicit and explicit warmth toward this person following dyadic interactions. The role of attractiveness in implicit warmth was more pronounced when the interaction required low (vs. high) self-disclosure. Moreover, explicit (but not implicit) warmth increased over time. These findings support a halo effect and the “familiarity breeds liking” hypothesis.