A job interview can be a good predictor of a candidate's enthusiasm, spontaneity, visual and verbal presentation, and ability to relate to others. It does not necessarily predict whether, and how well, they can actually do the job.
Interviews are normally rehearsed performances. A good actor can give you a false sense of what their performance will be like on the unrehearsed stage of day-to-day work.
Some people interview incredibly well. They are prepared, they have researched your company and they have a written list of questions. They have a highly polished presentation, excitement in their voice and display a strong confidence that assures you they can do the job. Maybe they can, but I have been fooled and disappointed on a number of occasions. The old adage that "practice makes perfect" applies to interviews as well.
On the other hand, some people interview poorly. They might be as prepared as the good interviewer, but are uncomfortable in what is a stressful position for most people. They don't show the confidence or enthusiasm you want to see. It takes longer to establish a rapport and, if you aren't patient with them, you may dismiss them prematurely.
If the job requires the candidate to use specialized knowledge or skills, solve problems, use the "tools of their trade", or has some other unique requirement, you need to do more than interview them. You need to use some method that allows you to evaluate those skills. If you had a Holodeck, like they do on the Enterprise, you could create a job situation and observe their performance; however, until that becomes a reality we need to use other methods to evaluate how they would perform on the job.
Traditional methods to evaluate job performance include testing, reference checking prior experience, requesting samples of work product and having the candidate complete a short assignment. Each method has its pluses and minuses and may require additional expert validation.
Testing allows you to determine how well the candidate uses the "tools of their trade". For example, if speed and accuracy of data entry are important to the job, you can have the candidate take a timed keyboard data entry test. As long as the candidate is comfortable in the testing setting, that should be a good predictor of their mastery of the tools they will use. If the candidate is certified in a particular knowledge area, that may also indicate they know how to use the tools they will need.
Reference checking will prove that a candidate held the positions they claimed, during the time frame stated. The law limits what information someone can disclose about a person, so you may not find out much more than that. On the other hand, references should be able to indicate a candidate's progression in an organization based on changes over time in their job title and salary.
Requesting samples of prior work product may or may not be a good representation of what the candidate actually did, since the product may be the result of the work of many people. You will need to determine what their specific role in its preparation was. Additionally, many companies insist on return of any company materials and intellectual property when an employee's job is ended, so they may not be able to easily produce prior work product.
Having the candidate complete a short work assignment is one of the best ways of determining what that candidate's performance will be on the job. This may be a "homework" assignment and you may need an expert to evaluate it. A candidate's willingness to invest the time gives them an opportunity to really show you what they are capable of. Their attention to detail will demonstrate how quality conscious they are. If you follow this assignment with a verbal presentation you will also be able to evaluate their effectiveness at presenting their ideas to others clearly.
If you are primarily concerned about how the candidate appears to present himself, you may be satisfied with an interview that lets the candidate describe how they would react in given situations. But remember that they probably learned the same interview techniques you did and will gauge their responses to what they believe you want to hear.
Relying on interviews alone will not produce the best candidate for the job.