It's always interesting to have them reference apps and designs that they think their app should end up looking like. Very quickly a general breakdown of things in regards to the app they are looking to build can follow. Even taking the approach of outsourcing and painting a picture of how that works.
A lot of times I've gone the route with at least doing the mockups of the pages with a little on how the UI will work. It's something that I know I'm selling that will be of value to them even if they choose not to have us continue on with the development and to just really boil down the idea and even more importantly what kinda of money and time it's going to take to develop the idea.