Hello friend,
You have asked a very good question!
Generally the interviews/ selection process we find that a person who is able to "impress" the most or one who is very well prepared gets selected.
In order to avoid this interview is structured with very "specific questions" which are relevant to the company culture, company issues and concerns.
Such specific questions make the performance based interview different from normal interview.
These performance based interview questions can improve the quality of selection. Also such questions can be sent through e-mail and based on responses suitable candidates can be short-listed.
This of course requires a lot of homework of understanding the job vacancy and very specific functional requirements so that very proper questions can be prepared.
Please refer a sample format enclosed. Here you would observe that we need to PREPARE specific questions for Performance Objectives of our company. [ for example instead of going by general assessment of the candidates cost consciousness or cost reductions he has implemented earlier - we ask him " We need to reduce our cost of following 3 items - what approach would you follow?". Such specific questions will allow you to judge the person better.
Hope I have been able to answer you.
Regards
Nishikant