While 'interactive' and 'student-centered teaching' are becoming buzzwords when promoting international English Medium Instruction (EMI) tertiary programmes in non-Anglophone countries, studies found that fewer lecturers possessed the competencies needed to teach content subjects interactively. This longitudinal case study looked beyond content lecturers' language proficiency, instead, addressing their interactional competencies based on the framework of 'Quality Interaction in Pedagogy'. The objectives were (1) to explore the interaction-related problems and behavioural triggers experienced by an untrained EMI lecturer of political science, and (2) to empirically assess the framework by observing classroom interactions before and after teacher training. The main participant was a non-native English-speaking lecturer (n = 1) teaching undergraduate students (n = 62) enrolled in three separate courses at Japanese universities. The study found that the framework was useful for measuring the effectiveness of classroom interactions after teacher training. The results also suggested that its systematic application done by extending Initiation-Response-Feedback sequences or broadening teacher language functions could enhance the quality of teacher-student interactions in tertiary contexts. These measures make a conceptually robust framework to help bridge applied linguists and content specialists on the issue of interactivity in EMI.