Rarely is one question on a topic enough - you must be prepared to follow up whenever you're not sure about what the person is saying by asking for clarification, details, and examples. It is important to avoid leading questions, that is, questions that because of the way they are worded or phrased indicate to the interviewee what sort of answer the researcher expects to hear. As a general rule, the researcher should interrupt the interviewee as little as possible. Things they mention that cry out to be followed up on can be noted and brought back up at a later point. However, if the interviewee strays drastically off course, it is appropriate to guide them back to the topic at hand.
Qualitative interviewers have to learn to tolerate silence.
It is important not to get impatient, but to give participants a chance to think about what they want to say. It is a natural human tendency to fill in pauses in conversation, and if the researcher can avoid doing so the interviewee will often fill the silence with more information. The interviewer should try to strike a balance between being formal versus casual. However, if the interviewer behaves too casually, the participants may not take the interview seriously and may stray off topic. Finally, it is important to leave the interview on a positive note and to get the interviewee to agree to be contacted again in the future if necessary.