Selectors who make up their minds prematurely can readily betray themselves to applicants through, perhaps, a less attentive manner and non-verbal communication. Would we not argument that conventions of politeness must be maintained to minimise the obvious problem of the superior-supplicant relationship. The interview is between two gentle-people. The interviewer avoids argumentativeness, attempts to trip up or embarrass the candidate, exhibit bombasticism or pursue a hidden agenda. Here is the theatre of the interview.
Politeness is the beign mask which conceals thoughts and judgments about the candidate.
The performance requirement is to maintain the friendly, interested facade. The interviewer need not declare their own impressions of the candidate. To do so would open up a can of worms as the interviewer is questioned to give an account of their judgments and mis-conceptions. The interviewer can remain dead-pan, the classic poker player. We can recognise also that politeness for the shy interviewer may mean that important questions go unasked leading to areas of a candidate's application not being explored thoroughly.