Many lecturers use teacher-centred styles of teaching in large undergraduate mathematics classes, often believing in the effectiveness of such pedagogy. Changing these beliefs about how mathematics should be taught is not a simple process and many academic staff are reluctant to change their ways of lecturing due to tradition and ease. This study describes the journey of a mathematician as he accepted the challenge to ask students to work interactively on well thought out questions in large lectures. The mathematician's espoused and enacted beliefs about lecturing were confronted through a cyclical process of developing questions, testing them in lectures, and refining them in collaboration with a research group. As he went through the process of testing and reflecting on his teaching practice, the gap between his espoused and enacted beliefs decreased as they became more aligned. The study demonstrates that the process of collaborative reflection with a team of educators can be a useful strategy for effecting change in lecturers' beliefs.