Interpersonal theories of psychotherapy hypothesize that the success of therapy depends on the therapist’s and patient’s dominant and affiliative behaviors as well as their interplay (complementarity). This study sought to investigate (a) how in-session interpersonal microprocesses predict postsession evaluations of the therapeutic alliance as well as cognitive-emotional processing and (b) how complementarity develops over the course of a therapy session. Twenty depressed patients were treated at a university-based outpatient clinic with a variant of cognitive therapy. One session was analyzed from each therapy’s middle phase using a novel real-time rating procedure (joystick method) to assess the patients’ and therapists’ affiliative and dominant behaviors as well as their resulting complementarity over the course of the session. The therapeutic alliance and cognitive-emotional processing were assessed by self-reports directly after the respective session. As predicted, more emotional arousal was associated with deviations from complementarity, whereas a positive alliance was related to affiliative patient behavior. Moreover, marginally significant trends suggest that refraining from answering to the pull of patient hostility might benefit both the alliance as well as cognitive-emotional processing. Overall, multilevel growth modeling revealed a significant cubic trend of complementarity over the course of the session. Implications for future research and practice are discussed.