Observing users without interjecting your opinions is crucial for unbiased prototype testing. Remain as unobtrusive as possible. Let the user interact with the prototype without interruption. For example, if a user struggles with a feature, resist the urge to explain it. Instead, ask about and note the difficulty they're experiencing.

Also, maintain a neutral demeanor. Avoid nodding or shaking your head in response to user actions, as this could subtly influence their behavior. Most importantly, save your questions and comments for after the user has completed their interaction. This prevents your input from altering their natural usage patterns.