Even the fastest sprinter of 100 meters race when passes the last .2 meter has to have the quality of mental agility and keeping nerves. No doubt sprinter has powered to reach finish line with power and practice but now before touching the finish line competition may dislodge and with fraction of seconds athlete may shift to 4th place.
PI is almost akin to finish line and on clearing this you will be declared selected and mission admission is achieved.
In Personal Interview it has been observed that invariably candidates with strong mental attitude perform better. Since you have already been tested on your domain knowledge in written test and later in GD on your skills on leadership, interpersonal quality so now you will be tested on the responses you offer to questions, non verbal body language etc.
Mental agility and holding your nerves with very positive attitude will be best option to clear PI.