Speak aloud. Describe the task you're given to complete, what you are doing, what you are seeing, and what you are trying to accomplish.
If something isn't where you were expecting it to be found, if you don't find something intuitive during your task, if you don't like something, talk about it! You're not going to hurt feelings. The customer wants to know what you really think of the site or app that you're testing. This helps them to develop their best product.
Keep in mind that you're working with prototype sites or apps, so not all buttons or menus do things. But, for the task you're working on, things should work just fine. (If not, speak up about it, what didn't work, what would you expect to see if it had, etc)
After your set of tasks is complete, there may be follow-up questions for you to answer. Make sure to answer them completely and be honest about what you thought of this experience.
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