Job candidates often form general impressions that they aren't suitable for a job when they have a short interview. While having a short interview may mean that the interviewer has no interest in you for the position, this is not always the case. Beyond your interview performance, several factors can affect the length of your interview.
In some interviews, the interviewers quickly become aware that you lack the basic education or experience preferred for the job. Often, this comes out in an application, but interviewers may overlook certain things or need to talk with you to decide. In many retail environments, on-the-spot initial interviews are common. This may mean that you are interviewed quickly following an application submission and before the manager has thoroughly reviewed your application. If you do not meet key requirements, extending the interview is unnecessary.
Sometimes an initial interview is short because it is the first step in a multiple interview process that gets progressively more involving. If an initial interviewer knows you would have one or more interviews to follow, he may simply try to confirm that you do meet basic qualifications and to get an initial sense of your fit for the job. Sometimes these first short interviews are called pre-screening or screening interviews.
An open interview process is a particular type of interview structure where employers invite all interested candidates to come apply and interview on a given day. This is common in retail, but it is used in other industries as well. Because open interviews are not individually scheduled and can draw a large group of candidates, they are intentionally and necessarily short. Typically, the interviewer is just trying to get an initial impression about you in order to decide whether to invite you for a longer interview.
Another reasonable justification for a short interview is that the interviewer has many interviews and limited time. Perhaps he would prefer one-hour interviews, but his schedule only allows each candidate 30 minutes or even less. Some interviewers will let you know ahead of time that the interview is going to be short. Other times, you will not know until you arrive or complete the interview. This is one cause of some candidates mistakenly assuming that a short interview is automatically a bad sign.