Common mental disorders, such as depression and anxiety, pose a major public health burden in developing countries. Although these disorders are thought to be best managed in primary care settings, there is a dearth of evidence about how this can be achieved in low resource settings. The MANAS project is an attempt to integrate an evidence based package of treatments into routine public and private primary care settings in Goa, India. Before initiating the trial, we carried out extensive preparatory work, over a period of 15 months, to examine the feasibility and acceptability of the planned intervention. This paper describes the systematic development and evaluation of the intervention through this preparatory phase. The preparatory stage, which was implemented in three phases, utilized quantitative and qualitative methods to inform our understanding of the potential problems and possible solutions in implementing the trial and led to critical modifications of the original intervention plan. Investing in systematic formative work prior to conducting expensive trials of the effectiveness of complex interventions is a useful exercise which potentially improves the likelihood of a positive result of such trials.