The qualitative changes in teachers' responses in challenging situations were analysed during a four-day Teacher Effectiveness Training (TET) course, which aimed at improving teachers' interpersonal dynamics with pupils, parents and colleagues. The participants were 21 teachers from one elementary and 23 teachers from one secondary school attending a TET course in Finland. Qualitative abductive content analysis was used to classify the data. Frequencies based on this analysis were also looked at. After TET the teachers described the behaviour of their pupils and expressed their feelings and the actual consequences of that behaviour, instead of using generalized labels and subjective interpretations of the pupils' behaviour. They were also more likely to support pupils' autonomy and agency by acknowledging pupils more, by increased listening or by asking pupils to participate actively in problem solving. This study deepens our understanding of the nature of the qualitative shift in teachers' thinking during interaction training.