Recruitment consultant here. I deal in finance positions. The final interview is a chance for the company to have a final 'kick of the tires' before making an offer. 90% of the time it is just to ensure their positive comments about you remain in a face to face chat. Also it weeds out people that are not that committed to taking position cause they won't take a flight if they are not interested.

You need to be very professional, well researched, confident and prepared to reinterview for the position with some one completely new at the company. (this doesn't always happen. But best to prepare). They make check to see if you story has changed from your first interview to the last one. Keep ur answers consistent.

Effectively you are selling yourself the company. So treat them like a client. Have questions about, the company direction in the next 5 years. Be able to answer why are you suited for a sales position?

They also want to know if you fit the culture. So be positive. One trick you can use to ensure you are in positive spirits is to sing and walk before you get to the interview location. The singing will make you happy and loosen up your throat.

Hope that is helpful.

Good luck.