Interviewing is a very time consuming process for both the candidate and company.  The predicament occurs when a company wants to hire top talent that is currently working, but still wants to make sure they are a good fit for the role.  In most situations, companies answer this question by requiring multiple interviews with various staff members so there are as many opinions in the decision making process as possible.  However, someone who already has a job will absolutely have a hard time fitting this into their work schedule. 
Passive job seekers do not want their current employers to know they are interviewing.  The reasoning behind this is simple: if they choose not to take the job, they want to know their current job is still secure.  Often, if a company knows an employee is looking, they will start to look to fill their position to be proactive, rather than reactive.  If they find a candidate and make that candidate an offer, the current employee that is interviewing will likely be fired. 
So how should a company interview a candidate that is currently employed?  Renee Mangrum suggests to not “waste their time!”  and further answers this question in her recruiting blog.  Read further, and you may find yourself attracting and reaching an offer acceptance with more employed candidates.