Like them, I was amazed at how many people got out their camera phones and actually paid attention to this.
They said "One thing we learned is that almost every human will immediately take out their camera when they see Darth Vader. We've done lots of crazy missions on the subway over the years, but I have never seen so many people taking photographs of our performance. How can you not take a photo of Darth Vader? It's impossible to resist!"
Hat tip to @manima for spotting this. Will someone in London put ImprovEverywhere's thesis to the test? Can we rely on Tube commuters to studiously ignore Darth & Co?