I certainly didn't mean to suggest that creating mock objects is free. In the second graph, I added an extra week to develop the application logic and the UI. O

I certainly didn't mean to suggest that creating mock objects is free. In the second graph, I added an extra week to develop the application logic and the UI. Originally these were scheduled to take 2 weeks, so 1 week is a pretty big chunk of time in comparison! I also call out this fact later, when I list some potential downsides to developing this way: "The cumulative time for the sequential method is seven developer-weeks, but the parallel method takes ten developer-weeks." The time to define interfaces and/or contracts should be a wash. If you design up front, then you can just use that, and if you are working in a more agile manner, your teams can communicate and experiment with getting the right interface. Either way, you have to spend time.