In line with the growing interest in the applicability of flipped learning as an alternative pedagogy to the traditional present-practice-produce model of English language teaching, this study investigated its effects on adult learners’ development in oral proficiency. Twelve intermediate-level Korean learners of L2 English participated in an intensive language course comprised of 24 sessions for a three-month period; six of them were assigned to a traditional group where they learned in a typical teacher-centered classroom setting while the other six participated in flipped learning. Learners with similar backgrounds and attitudinal profiles were chosen as a focus group and their achievements and classroom behaviors were compared in pairs. The results indicated that the students in the flipped class outperformed their counterparts in the traditional class on both immediate and delayed post-tests. The flipped class also demonstrated some behavioral characteristics that were not observed in the traditional class, such as connecting prior and current discussions, formulating more elaborated and personalized questions, and exercising autonomy by proposing discussion topics.