In our study, we aim to investigate the relative importance of verbal, paraverbal, and nonverbal behavior in first impression formation, specifically in relation to perceptions of warmth, competence, and morality. We analyze 90-second videos of speakers introducing themselves and use both human raters and artificial intelligence (AI) algorithms to rate the behaviors displayed. The results show that while nonverbal behaviors are most strongly associated with warmth impressions, verbal and paraverbal behaviors are more strongly associated with competence and morality impressions. We also find that morality impressions are the least predictable from thin slices of behavior, and the behaviors most important for understanding impression formation differ for warmth and morality. The limitations of our study include its observational nature, which precludes establishing causal relationships, and its focus on a specific population and context, which may limit its external validity. Nonetheless, our study's use of dynamic behaviors and AI-facilitated analyses offers a novel and promising approach for future research on the dynamics of social cognition.