Global policy on HIV prevention among marginalised populations recommends a community-based approach with participation and mobilisation as central features. The overall aim is to empower individuals and groups to reduce their vulnerability to HIV. Community empowerment initiatives have had mixed results, however, in spite of utilising very similar peer-education based intervention strategies. This is particularly true of community-based interventions in sex work settings. Drawing upon an ethnographic study conducted in the early years of a well-known sex worker initiative-the Sonagachi STD/HIV Intervention Project (SHIP) in Kolkata, India-this paper argues that ethnographic research can illuminate the complex and inter-dependent dynamics of context, practice, agency and power that are specific to a project and shape the course of intervention implementation in ways that may be 'hidden' in conventional techniques of project reporting. Two detailed excerpts of what we refer to as 'private contexts of practice' are presented-focusing upon the complex processes underlying community mobilisation and peer education. We show that the gathering of ethnographic forms of evidence in conjunction with more conventional evaluation measures has two distinct benefits. First, an ethnographic approach is able to capture the play of power through observation of real-time events that involve multiple actors with widely different perspectives, as compared with retrospective accounts from carefully selected project representatives (the usual practice in project evaluations). Second, observation of actual intervention practices can reveal insights that may be hard for project staff to articulate or difficult to pinpoint, and can highlight important points of divergence and convergence from intervention theory or planning documents.