Purpose: Despite calls for studying interaction processes in coaching, little is known about the link between coach–client interactions and coaching success. In particular, interpersonal behavior in coaching remains unexplored, although it is considered highly relevant to social relationships and interaction outcomes. This study takes first steps to address this gap. Design/Methodology/Approach: We examined the dynamics of coaches’ and clients’ interpersonal behavior based on the two basic dimensions affiliation and dominance. Furthermore, we investigated the link between emergent interpersonal behavior patterns and coaching outcomes. To this end, we videotaped and analyzed a total of 11,095 behavioral acts nested in 30 coach–client dyads. Findings: Sequential analysis showed that reciprocal friendliness patterns were positively linked to working alliance. Coaches’ dominant–friendly interaction behavior particularly activated clients, in terms of showing dominance during the coaching interaction process. Clients’ dominance was linked to their overall goal attainment. Implications: Our results highlight the importance of interpersonal behavior for coaching success. Specifically, our findings suggest that dominance interaction patterns are context- and relation-specific, offering an explanation for contradicting empirical studies on interpersonal dominance. For coaches, our study implies that high awareness for interpersonal signals can help establish a positive atmosphere and activate clients’ dominance. Originality/Value: This empirical study uses behavior observation and interaction analysis to understand the interpersonal dynamics during coaching sessions. Our results increase our theoretical understanding of coaching effectiveness by shedding light on the micro-level behavioral dynamics that drive successful coaching processes.