Interviews have a likelihood of going in unexpected ways, it is helpful to have a topic or list of questions to refer back to when the interview needs to steer back to the original objective. Include everything you would like to know from the user and keep questions open and dialog-provoking for maximum effectiveness. Avoid questions that can be answered by a simple yes or no by framing them in a way that encourages the user to expand on questions by using phrases that include When, Where, Why, What, Who and How.