What are some effective ways to test your service design prototypes with real users?

I agree, and not to add complexity, but you can even create a few mini-prototypes to demonstrate the differences between different key dimensions of your service. While designing a new training program for leaders in a particular government office, we created a prototype consisting of 3 options of increasing levels of commitment, depth of learning, and connection to peers -- three elements that we knew were important to learn about to inform progression. We showed these options side by side during our testing sessions and repeatedly asked "Why?" on peoples' preferences to dig deeper into what mattered most to our learners.

Absolutely! It is essential to choose the correct prototype. It doesn't have to be complex but has to serve its purpose. Also, a good prototype is not necessarily expensive. A low-fidelity product prototype made of foam or cardboard may be enough!

I'm often surprised with how far just paper, markers and tape can go, when prototyping Especially in the beginning, there is often no need for high fidelity prototypes. Most likely, you just want to test basic assumptions and immediately see big challenges. And for this quick pen and paper, prototypes can go a long way. Just as a heads-up, remember to put in enough effort into how it looks. Even quick n dirty, should still be understandable. This is something I forget, although often late to messy untouchable handwriting and prototypes, for example. Have fun prototyping!

Designing a prototype and planning a testing session are not necessarily 1-to-1 activities. In other words, you may create a detailed prototype (regardless of fidelity) that would take 2 hours to fully review/test with your end user, but you can only get 1 hour with them to test it. In this likely scenario, spend time carefully reviewing the key components of your prototype. Answer the question: What do we absolutely need to be true / need to know about this service without which we cannot proceed (to higher fidelity, to pilot, etc.)? Test for those elements. The constraints may increase your creativity and improve your results.

You definitely should have a script but it is important to also follow the user where they go naturally - a script is more of a reminder for you to make sure you cover the relevant points, rather than a strict order of questions.

If testing the prototype with a larger audience, surveys and questionnaires are an efficient way to go about it. Develop questions that cover various aspects of the prototype, such as usability, functionality, and overall satisfaction, but keep them a mix of objective and open-ended. Distribute these surveys to users after they have interacted with the prototype. Analyzing the responses can highlight common trends and pinpoint specific areas that need improvement.

In my experience, synthesis is equal parts art and science. For some people it's more intuitive than for others, but anyone can improve with practice. You'll find as you conduct synthesis that there are numerous ways you could cluster and label the data that lead to different insights. Allow time for your team to reflect on your clusters + themes to see if anything interesting or surprising comes up. If your themes reveal nothing new, no interesting insights, see if re-organizing the data reveals anything more interesting or surprising. If there's still nothing new or interesting, you may need more testing, you may not be asking the right questions / testing the right components of your service, or your prototype may need adjustment.

Personally, if the service is fairly simple, you could try to enact/stage it for a small group as well. And to get going quickly, even paper prototypes work fairly well. Since the aim is to understand how the user behaves and feels, the medium is not as important as the communication. So choose the easiest tool and spend more time on the execution-related parts of the prototype.