I recently gave my third UPSC interivew(CSE-2015) and I have seen my name twice in the final rank list of Civil Services Examination, so my understanding of the Personality Test is based more on to the first hand experience(s).
Let me give you a realistic picture of Civil Services interview:
Firstly if I could describe CSE interview in one word then it would be "random". Now why is that? For the simple reason that people who are sitting there are not professional interviewers. Sure they are very experienced people and come from diverse backgrounds but is the life experience a sufficient criteria to make one a good judge of personality. If that's true then all HR firms would have only hired 50+ aged people but they are not. Since the people there are not professionally trained for the job hence interview outcome can be very random with marks of same person dwindling tremendously in successive years.
Secondly interviewers have their own biasesand hence their judgement is not necessarily completely objective. There used to be one board earlier whose chairman generally disliked people from technical background ( specially IITs and NITs) coming to civil services.
Thirdly not every interviewer takes his/her job that seriously. There have been boards like that of an infamous lady member who would leave the room during interview or sometimes have a lavish meal in between the interview. Then there was one ex-UPSC member who was playing games on i-pad during one of the interview.
Forthly not every UPSC member is best suited for the job. Most of them are placed there because of political connections. A.P. Singh was a UPSC member and chairman of interview board who was "judging" candidates whether they are "honest" and "ethical" enough to join civil services. However he was himself accused in corruption cases and had to leave the position at UPSC because of case against him in Enforcement Directorate. Ex-CBI chief AP Singh resigns as UPSC member. Then there was one chairman (ex-UPSC member) who in one interview was caught unaware of the fact that Indian Information Service is also one of the civil services which people opt via Civil Services Examination. So at times interviewers are themselves clueless about many things.
There are people saying that its only a a test of personality and not of knowledge and you are judged only on the basis of questions you answer. However I differ with this opinion. Some boards like that of Alka Sirohi ask highly factual and technical questions and there is hardly a chance of getting good marks if you fail to answer such questions ( some exceptions are always there).
Some things which I have learnt from my interviews is that it is good to have a decent command over English language and being eloquent helps but don't overdo it. Also remember that the board members are alpha males ( or females) so be very respectful and humble. Even if you engage in an argument with them then be polite and if it stretches too far then take a dignified exit rather then pestering to prove that you are right ( even if you actually are).
I don't want to discourage anyone but one should be aware of reality. Truth is that UPSC is a government organisation and it is as efficient and effective as any other govt. department can be. So don't have an idealistic pretty picture of an interview where the members will be at the edges of their chairs to listen every word coming out from your mouth. For you it might be the most important day of life while for them its just another routine day. In my interviews I have seen people yawning, dozing and asking questions which are factually wrong( yeah one of the interviewer thought that Dadabhai Naoroji was a historian).
One more thing that interviewers except the board chairman( who is a UPSC member) are specially invited for every session. They come from diverse backgrounds like bureaucrats, professors, academicians etc. Now they might be expert in their subject but many of themhave no idea what to ask from civil services aspirant which leads to some absurd questions being ask
