An interview is in contrast to a questionnaire best used when you only want to include one or a few participants, but get much more detailed and informative answers. There are two different styles of interviews each with their strengths and weaknesses, and you must carefully evaluate which one is the most suitable in your case.
An open interview is not planned in advance. The interviewer talk with the participant and tries to direct the discussions towards what the interviewer wants to know more about. In open interviews, it is important to avoid leading questions (questions with a simple or yes/no answer). Questions shall be open to allow for the discussion to continue. Open interviews have the advantage that the participant is in control and talks about things he/she thinks are important, not what the interviewer thinks are important. The disadvantage is that open discussions can be difficult to manage and often require experienced interviewers.
A closed interview is planned in advance. The interviewer has prepared a fixed set of questions he/she asks during the interview. The advantages are that it is easy to repeat the interview with new participants and that it is much easier to manage than an open interview. The drawback is that the prepared questions might not be seen as very important by the participants, who might think other things are more relevant. The answers from closed interviews are also much easier to analyze.
Regardless of interview style, it is recommended that you record the interview. Without recordings, you cannot go back and see if you missed anything important.