Hi
mjmhtjain ,
The essay writing is basically to test you communication and articulation skills. So there is no specific agenda for essay writing. You may be asked to give your opinion on an ongoing HOT topic in the world/country, or you may be asked to justify your stand for a statement given statement etc. You dont have to specifically prepare for essay writing in my opinion.
For the PI round, you will mostly be asked from whatever is mentioned in your CV. If you know well about whatever you have mentioned in your CV, then your half work is done. There might be situational questions to check how well you handle/manage a particular situation etc.
Apart from your communication, your confidence, attire, attitude etc. will also be considered as a part of the overall profile-based interview round.
All the best !!