01 See Snyder, C. (2003). Paper Prototyping: The Fast and Easy Way to Design and Refine User Interfaces. Morgan Kaufmann.
02 See Walker, M., Takayama, L., & Landay, J. A. (2002). “High-fidelity or Low-Fidelity, Paper or Computer? Choosing Attributes When Testing Web Prototypes.” In Proceedings of the Human Factors and Ergonomics Society Annual Meeting (Vol. 46, No. 5, pp. 661–665). SAGE Publications.
03 Of course, you can always temporarily lift that rule to have the operators help the user.
Paper prototyping is a common low-fidelity method to prototype and test software and interfaces using interactive paper mock-ups.  The different screens of the interface are hand sketched on paper and presented to a user. The user can then use the interface by “clicking” with her finger, indicating what she wants to do. A researcher simulates the operation of the computer or device simply by replacing the screen page with the next one or by adding details on smaller pieces of paper onto the sketch (e.g., to add pop-ups).
Paper prototyping has been part of the tool set in prototyping software and interfaces since the early 1990s and rightfully earned its place. The main reason for the success of this method is that – especially early in the process – the interfaces are much faster to build on paper than using digital mock-ups, let alone programming. Plus, they are easy to change as well, even during the test of the prototype itself. Try this with code.
In addition, research comparing low-fidelity paper prototyping against computer-based, high-fidelity prototypes has found that “low- and high-fidelity prototypes are equally good at uncovering usability issues.” Even though a paper prototype is quite lo-fi in its basic appearance, it can be high-fidelity in other aspects, like the navigational structure or the actual set of features, thus delivering deep insights for these areas early on.
Of course, there are limitations. Medium-specific problems, for example, cannot be tested. Many paper prototypes also deliberately leave out most of the look and feel. However, paper prototypes are still especially helpful when exploring different design directions. High-fidelity prototypes, on the other hand, play their strength when it comes to actual look and feel, true performance data (responsiveness or latency of the application), or presenting the prototype to management or other stakeholders who are not familiar with low-fidelity prototypes.
Sketches of wireframes are a great starting point for paper prototypes. Wireframes give you a good overview of the layout of the site or application – but they often do not contain real content, and instead use placeholders rather than real images or copy. This makes it harder for the audience to use them in test scenarios since a lot of (too many?) gaps are left for the user to fill in. So, start with the wireframes and quickly add key content.
Another intriguing aspect is the impact of this method on decision making. Paper prototypes are a minor investment and clearly created to be thrown away. This makes it easier for those who created the prototypes to let go and embrace necessary changes. Similarly, actual users taking part in the test tend to feel more comfortable about suggesting changes.