This paper analyses a case study which employs qualitative research methods, utilizing a case study strategy, to delve into the relationship between the beliefs and classroom practices of a second language teacher, Dantes. The study integrates semi-structured interviews and classroom observations to explore the teacher’s beliefs and gather data on classroom practices. The interviews not only reveal the teacher’s initial beliefs but also uncover deeper convictions through final interviews, such as the unconscious belief in using a step-by-step teaching method. Six one-hour classroom observations enhance the validity of the observational data and provide an assessment that goes beyond the teacher’s self-interpretations of behavior. However, the observation design may be influenced by the researcher’s subjectivity, and the use of a single observer may increase the personal subjectivity affecting the outcomes. The study also employs data triangulation, combining data from interviews, journal writing, and final interview reflections, to strengthen the validity of the research findings. Additionally, member checks further validate the results through Dantes’ feedback and reflections on the research outcomes.