An interview in a volunteer recruitment setting shouldn't have the same formality as in a formal job setting. But it does share a lot. You may carry out interviews of your applicants without realizing they're interviews! Some organisations called them 'get to know you meetings' or 'coffee meetings'. Whatever the name used, if you formally meet with your applicants, and use that meeting as part of your evaluation process for deciding on their application to your organization, you're essentially carrying out an interview. See also, How do I record the outcomes of interviews with applicants?