The paper presents findings from a research lesson in theatre as a school subject at upper secondary level in Sweden. The research lesson is one of five and is part of a larger design in a research project with the aim of developing theatre teaching practice. In the transcripts of filmed material, we follow how the teachers are using variation theory as teaching theory. In planning the research lesson, the teacher and the researcher used variation theory, starting with the patterns of variation contrast and generalisation. During the research lesson the pattern of variation fusion was used as an answer to both the students acting in an improvisational exercise and the students observing and reflecting on what happens in the improvisation when different patterns of variation are used. The research lesson is an example of using variation theory both when systematically planning teaching and when analysing spontaneous feedback in the teaching situation. This could lead to discussion and challenging of a theatre teaching practice described as being part of a tacit knowledge tradition.