Agency AOR selection criteria

We are helping a digital agency try to move from DOR to AOR - they want to own the brand development. The question is how do clients choose agencies? What are the drivers of selection? The stated and the unstated ones. Our client wants to win more clients and defend against strategy consultants coming into their space .. Accenture has one of the largest digital agencies now and they have C suite relationships. Our client is also looking for AMAZING ideas and strategy to help them win more clients. A key hypothesis is that clients wont trust a digital agency to do brand/creative work. Typically the digital agencies take the creative form the AOR and make it digital. Our client wants to become that creative agency, they have hired the talent and now need to figure out how to win the business. We are going to interview 500 clients (marketing, procurement, c-suite) and we need background research ahead of developing our qualitative discussion guide and quantitative survey.