There is a growing understanding that teacher education should not be handled either exclusively by the University or in School settings. Nevertheless, the issues on how to foster a fruitful partnership between these two different institutions remains an open and challenging point. The practicum is a critical point in the pre-service teacher's development, usually because it is the first time they have to face the concrete professional issues, which they only had a partial view of as students. This research is a year-long inquiry on practicum in the Physics teaching program developed in the Institute of Physics at the University of São Paulo, Brazil. Drawing on cultural-historical activity theory, contradiction is used as an explanatory principle to examine change and development in the practitioners' activity. Therefore, the research includes in the analytical framework the contradictions and the processes of overcoming it. Examples are provided in which there is little control therefore demanding extra efforts but makes room for active participation. The data analysis indicates that the source of agency and activities transformation is precisely a contradictory process, which might be overlooked. The pre-service teachers' agency is shaped by contradictory process within the activity -- practicum -- rather than aligned forces pushing it toward autonomy, student sensitiveness or professional identity.