<p>Understanding the nuanced mechanisms of effective patient-therapist dialogue is crucial for advancing psychotherapy research and practice. This study examines the role of discourse diversity – an understudied factor – in patient adherence to therapy. Using the MATRIX, an evidence-based coding system that transforms full session transcripts into coded sequences, we quantified discourse diversity both within sessions and over the course of therapy. Our dataset comprised 265 sessions from 53 short-term psychodynamic therapies, each scheduled for one year. The number of sessions completed during this prescribed course of therapy was used as an indicator of patient adherence to the therapy process. Based on the median treatment length, therapies were categorized into short- and long-treatment groups. Linear regression models examined changes in discourse diversity over time and their association with treatment length. Results showed a significantly greater increase in discourse diversity in the long-treatment group, suggesting that sustained commitment to therapy is linked to evolving dialogue. Moreover, patient and therapist diversity levels were highly correlated, indicating a mutually shaped, reciprocal process. Additional analysis comparing observed and simulated therapies revealed a distinct discursive pattern unique to psychotherapy. These findings suggest that an evolving and dynamic dialogue may be important for improving patient adherence to therapy. They provide preliminary support for using discourse diversity as an indicator of treatment progress, with potential applications for optimizing psychotherapy processes and assessing therapeutic progress in research and clinical settings.</p>