We built Series, a visual campaign builder that allows customers to easily create sophisticated customer journeys using an intuitive drag-and-drop interface. This was achieved after considering four different approaches: evolving Smart Campaigns, creating a rigid visual builder, a semi-flexible visual builder, and a fully flexible visual builder. Our guiding principle was "No black boxes," ensuring customers knew what was happening in each part of the series without ambiguity. We looked outside our product by analyzing customer feedback, competitor solutions, and digital visual builders to guide our thinking. Finally, we tested our assumptions with dozens of customers to discover dozens of use cases, needs, and confusions that hadn't been previously considered. The flexible visual builder was chosen as it ticked all boxes: understandable, resonating with the mental model of most people in the market, and testing well. This process taught us lessons on iterative improvements, considering external perspectives, and investing in realistic prototypes to achieve objective confidence in our solution.