There are many possible scenarios in which a candidate should decline a job interview offer, including dissatisfaction with the salary, dissatisfaction with future coworkers, or dissatisfaction with the work itself. Many of these scenarios can be mitigated through careful discussion and negotiation during the application process.
Part of the dissatisfaction with either the salary or the job itself may come from receiving a competing job offer shortly before the call for the interview. If the first job is clearly better than the prospective one offering the interview, then the candidate should simply decline the interview. If the candidate does not favor one job over another, then having a competing offer may help with negotiations regarding salary and job benefits.
Research is an important step before accepting an interview offer, and during research, the candidate may discover something about the prospective job that he seriously dislikes. The candidate should decline the job interview if this has happened.
If the job offer involves multiple interviews, it is possible that the candidate has discovered something about management or other staff that he is uncomfortable with. It is important to politely decline further job interviews if this is the case. Finally, during either research or prior interviews, the candidate may discover that the prospective job has an uncertain future. In this circumstance, he should decline the interview.