Evidence suggests that extraverted (i.e., bold, agentic) behavior increases positive affect (PA), and could be targeted in wellbeing interventions. However, this evidence is either causally ambiguous or has questionable ecological validity, and the potential costs of sustained extraverted behavior have received minimal attention. To address these limitations, we conducted the first randomized controlled trial (RCT) examining the wellbeing benefits and costs of an extraverted behavior intervention conducted in everyday life. Participants (n = 147) were randomly assigned to an “act-extraverted” intervention or a “sham” (active control) intervention for one week in everyday life. Additional data for a contact control condition were obtained from a previous study (n = 76). Wellbeing outcomes included PA and negative affect (NA), feelings of authenticity, and tiredness—assessed both in the moment and retrospectively. There was a positive overall effect of the acting extraverted intervention on PA and authenticity. However, wellbeing outcomes also depended on dispositional extraversion: more introverted participants had weaker PA increases, experienced increased NA and tiredness, and decreased feelings of authenticity. Implications for wellbeing interventions and personality theory are discussed.