Students' interest is considered an important learning outcome, but it is also a relevant predictor for student learning, and future vocational choices. According to numerous studies, however, students' interest in STEM fields usually declines during the course of secondary education. From the perspective of science education, it is therefore necessary to foster or at least maintain students' interest. Despite the variety of approaches that have already been examined in order to promote student interest, the problem of low-interested students remains. Prior findings indicate that specific person characteristics and the students' perception of the situation seem to be moderate the effectiveness of many approaches. The current intensive repeated measure intervention study addresses the investigation of a possible interest trigger (formative assessment) and also the process that influences the perception of this trigger. Based on a sample of 9th-grade chemistry students (N = 200), three different interventions of formative assessment were implemented in regular classrooms. Students' situational interest was assessed repeatedly in short time intervals. Based on multilevel analyses, not all interventions were perceived as equally interesting by the students. While students' individual interest influences the perception of all interventions positively, the impact of gender, chemistry grade, and enjoyment varies across the interventions.