1. Dress formally.
A plain or stripped shirt with Plain Pants, Tie and Shoe are good for the men. No Jeans and T Shirts Please. If you go with a T Shirt that says "hit me", the interviewer may do that (with your marks). The time tested attire for ladies is the saree. Some say that Cotton sarees are better.
2. Reach the Destination in time. This is a very important point to note. To fumble and search for your certificates and hall tickets is going to do no good for your cause.
3. In most cases, the interviewer in the centre of the panel (the chairman) will be a non medical man (TNPSC Member or from IAS cadre) and the most important fact is that the marks will be often with him/her. So no matter how well you answer the rest (subjects) if you "irritate" the chairman (with your GK performance) you are in trouble
4. Please wish a Good Morning or Afternoon and for heaven sake do that with both your hands in a "prayer posture" and then sit only after you are asked to sit. There have been instances where candidates have just told good morning and the interviewer told replied to them that the manner in which the candidate had wished is the procedure for wishing colleagues and to wish seniors he/she has to wish like this (there was a demonstration) and the interview result was what we expected
5. In most cases your hands will be free (from certificates and other files) and hence a proper “வணக்கம்” is not a problem