Customers will give you much more information when they feel comfortable with you. After all, you may be asking them slightly personal questions such as why they buy Brand A over Brand B, and you will need to establish a comfort level with them before they will freely give out that information.

A good interviewer will make each customer feel welcome and comfortable in order to get to know them in a limited amount of time. This is very difficult if they are nervous, stressed out, or guarded.