We’d like to avoid spending time for making (interactive) prototypes at early stage of the project. We should have a few solid ideas from our numbers of sketches to experience its interactions. It doesn’t have to be all the functions, but the some of flows that sketches or static wireframes cannot express.
iOS Human Interface Guidelines (http://bit.ly/1SMqged) says:
In the very early stages of your design you can use paper prototypes or wireframes to lay out the main views and controls, and to map the flow among screens. You can get some useful feedback from testing wireframes, but their sparseness may mislead testers. This is because it’s difficult for people to imagine how the experience of an app will change when wireframes are filled in with real content.
You’ll get more valuable feedback if you can put together a fleshed-out prototype that runs on a device. When people can interact with your prototype on a device, they’re more likely to uncover places where the app doesn’t function as they expect, or where the user experience is too complex.