Ask Users To Talk Through The Process And Observe

When users are exploring the prototype ask them to say their thought processes out loud so you can see how their mind is working and observe more closely. This might take some getting used to for some users - but it will help you evaluate the intricacies much closer.

Observe how users either "correctly" or "incorrectly" use the product - and resist the urge to correct them! You want to make sure your product is falling in line with intuition - so don't explain or interfere. Watch and see where different facets are either failing or succeeding. And remember - the user is always right.