In essence, the easiest way to remember primary vs. secondary questions is to recall that primary questions are asked to start or begin a new topic or thread of conversation. (Hence, primary!)
For example: Tell me about your relationship with your boss.
This question introduces the idea of how an individual works with a supervisor. Based upon the response received, the individual may be asked a different question that goes deeper… also known as a secondary question. That is to say, secondary questions tend to follow-up on the primary question and its related information.
For example: You mentioned that you liked having a lot of freedom on your schedule. Please elaborate on that for us.
By taking a topic that was brought up by the interviewee, the secondary question follows up on the idea and allows for more information.