Your real competition isn't some job hunter off the street. It's the candidate who was coached by The Headhunter. From The Archive 20. Don't conduct junk interviews.
Employers routinely waste time in interviews by asking the wrong questions.
Legitimate questions are specifically about the work and about the candidate's ability to do it. These need to be asked first, because the answers will determine whether the interview should continue. Junk questions are ones the manager feels he should ask either because they're on a list the personnel department gave him, or because he's heard they're popular questions. Some questions may be clever or interesting. But, if they're not about the work, put them aside until you've determined the candidate can do the job.