Professional learning is intended to improve teaching, benefit student learning and in the longer term improve achievement of targets in schools. When teachers take responsibility not only for their own learning but also for their colleagues', it can lead to a shift in attitudes towards collaborative learning. This study presents an example of a professional learning community (PLC) based on teachers' engagement in systematic inquiries. The aim of the study was to explore crucial factors during the process of critical friendship that could be related to a PLC. The research context was a series of shadowing sessions where teachers were expected to document teaching and provide feedback. The analyses are mainly based on documentation, in the form of shadowing logs produced when teachers study each other in action. Findings indicate that the role of critical friend was hard to internalise and that most of the teachers did not use it to its full potential for learning, according to the Vygotskian concept of the zone of proximal development. Three crucial factors for learning in a PLC are addressed: active choices related to teachers' and students' learning during different phases in the inquiry; the complexity of mastering several skills in action research at the same time; and the teachers' attitudes to professional learning.