01 For example, see Hallgrimsson, B. (2012). Prototyping And Modelmaking for Product Design. Laurence King Publishing.
02 For example, early in the process, a six-hour prototyping session could push a team of 3–5 people to produce 20+ sketches and 3–5 desktop-sized cardboard prototypes before making a decision and building one full-sized one in the last two hours.
03 For an example where service designers set up a cardboard hospital, see Kronqvist, J., Erving, H., & Leinonen, T. (2013). “Cardboard Hospital: Prototyping Patient-Centric Environments and Services.” In Proceedings of the Nordes 2013 Conference (pp. 293-302). A video is available at Cardboard hospital: https://vimeo.com/46812964.
04 Of course, you can always temporarily lift that rule to have the operators help the user. You can consciously decide to enable a team discussion if this becomes necessary during the process – for example, over a roadblock that cannot be solved right away.
Cardboard prototyping is a common low-fidelity method to prototype and test physical objects and environments that are part of a service experience – for example, the interior of a shop environment, a ticket machine, furniture, devices and smaller props, and so on.  The prototypes are built quickly, using cheap paper and cardboard mostly. Other equally easy-to-use materials like foamcore, plasticine, or duct tape often complement the mix of materials.
Depending on the scope, the prototypes can be small-scale, actual size, or even bigger than life. To further explore and validate core functionality and the role of these objects in the context of the future service, cardboard prototyping is often used in conjunction with or as part of walkthrough approaches (e.g., desktop walkthrough or investigative rehearsal).
Prototypes made from cardboard are cheap and easy to make. Cardboard prototyping indeed has one of the lowest entry barriers of any of the prototyping methods. Almost everybody has done this before, either as a kid or as an adult helping children. Just like paper prototypes, cardboard prototypes are clearly created to be thrown away. This makes it easier for those who created the prototype to let go and embrace necessary changes. Also, actual users taking part in the test tend to feel more comfortable about suggesting changes.
The most important part of cardboard prototyping is the process of prototyping itself. It helps to concretize the initial concept and explore its details, strengths, and weaknesses. A great way to start is to build many smaller scale versions before switching to full size, for the simple reason of speed. 
Scale models also set the stage for small-scale experience prototyping techniques like desktop walkthroughs, as you literally build the space and key artifacts to enrich the walkthrough experience.
Full-size models help to set the stage for immersive experiences like investigative rehearsal or process walkthroughs.  They encourage and enable a deeper exploration and iteration of the design. A great example for this comes from Chick-fil-A, which uses cardboard prototyping to test the setup of a whole restaurant. New setups are built in foamcore (including walls, tables, coffee machines), then rehearsal techniques are used to test the flow and the experience with the design team, operators, and architects.
Cardboard prototyping follows similar steps to paper prototyping, replacing the paper prototypes of mostly 2D interfaces with more generalized 3D physical models (that in fact might contain paper prototypes within). Just like a paper prototype, a cardboard prototype is used by a test user to accomplish given tasks while an operator manipulates the different parts of the prototype to simulate the functionality of the object.