Much attention has been placed on the need to develop and evaluate complex interventions targeting public health issues, such as reproductive health. However, and as has been the case in the recent past, even well-designed trials will be flawed unless meticulous attention is paid to ensuring the most appropriate intervention is designed and developed. This requires a well-resourced and carefully planned feasibility study, incorporating both formative and process evaluation, with particular attention being paid to the context of the proposed intervention. In this paper, we describe the way in which a feasibility study helped redesign and shape a complex intervention targeting adolescent sexual health in rural Zimbabwe. By using a mixture of in-depth interviews, focus groups and participant observation with pupils, parents, teachers and education officers, we were able to show that the intervention as originally conceived was unlikely to be deliverable. Process evaluation findings from the feasibility study led to substantial changes to both the content and delivery of the proposed intervention, which is now subject to testing for effectiveness in a large community randomized trial.