Professional designers and developers often struggle when transitioning between the design and implementation of an interactive system. We conducted three studies that focused on the design of custom interactions to understand the mismatches between their processes, tools and representations. We found that current practices induce unnecessary rework and cause discrepancies between design and implementation. We identified three recurring types of breakdowns: omitting critical details, ignoring edge cases, and disregarding technical limitations. We propose four design principles to create tools that mitigate these problems: Provide multiple viewpoints, maintain a single source of truth, reveal the invisible and support design by enaction. We applied these principles to create Enact, a live environment for prototyping touch-based interactions. We conducted two studies to assess Enact and to compare designer-developer collaboration with Enact versus current tools. Results suggest that Enact helps participants detect more edge cases, increases designers' participation and provides new opportunities for co-creation.