Behavioral interventions are proposed as a critical treatment component in psychotherapy for personality disorders. The current study explores behavioral interventions as a mechanism of change in Metacognitive Interpersonal Therapy, an integrative psychotherapy for personality disorders. The goals and implementation of behavioral principles are illustrated through the single case study of Roger, a 57-year-old man diagnosed with avoidant personality disorder and depressive personality disorder. Transcripts of interviews and therapy sessions illustrate the role of behavioral interventions, including behavioral activation, in Roger’s treatment. Roger demonstrated a reliable change from baseline to posttreatment across all measures. He also showed gains with regard to his occupational functioning, interpersonal relationships, and sense of fulfilment. Implications with regard to treatment planning for personality disorders are discussed.