Hi
Insist on checking supervisory references to the candidate: Candidates sometimes say they don't want you to contact a former or current supervisor. There are two common reasons for this reluctance.
The first reason is that they don't want their current boss to know they're job-hunting. They don't want to burn any bridges with their current employer until they know they have a new job lined up.
The second reason is that they're afraid their current supervisor will say something negative about them.
If you're seriously interested in the applicant, you should tell them that they are in the final list but that you must have a full picture of their work history, and therefore must check with their past and current supervisors, even if the applicant has provided the names of other individuals in the organization as references. Explain that if you can't have a discussion with the immediate supervisor, you may not have enough information about their candidacy to make an informed decision, and you won't be able to consider them further. Then ask them to explain their reluctance and work through it with them.
If their current boss doesn't know they're looking, give the candidate some time (a day or so) to go back and have their own conversation with their supervisor, so that your reference call won't catch the boss off-guard.
If they're afraid the supervisor will say something bad about them, ask them to describe what they think the supervisor will say -- and why they think s/he'll say it. Sometimes supervisors will say discouraging things, even about good employees, if they're trying to keep the worker from leaving. Sometimes if there are problems in an employment relationship, the problems are with the boss -- and not with the employee. And sometimes there's legitimate negative information that the candidate would rather you not hear.
If the candidate thinks you may get negative information from a current or former supervisor, assure them that their own explanation of the situation will also help you weigh what you hear. Encourage the applicant to give you the names and phone numbers of others in the organization who might be able to give countering or balancing information.
cheers,
Rajat