Luckily, we had a product owner with a detailed vision that just needed to get out on paper. Several all day, and sometimes night, sessions to get the ideas out of his head, on to a white board and on to user stories and wireframes. In conjunction with user research, user surveys and simple user testing we had the requirements needed to start working on a prototype.
While iterating on wireframes and clickable flat mockups does provide valuable insight, there is nothing like a clickable prototype that feels real to the user.
Built using bootstrap and themed with HTML/CSS/JS and populated with paux data and behavior.
Used for demos, presentations, and user testing.