Leave the Interview well

Never apologise at the end of an interview. Even if you were a few minutes late arriving do your apology at the beginning, no need to remind them of it now.

Do smile at them as you leave the interview even if you feel like a Cheshire cat.

Act as if it has been a good interview. Say thank you to everyone. Say goodbye to everyone. Shake hands. Look at people and leave with a spring in your step.

Do all of this even if you think the interview has been a disaster. How you feel about your interview is unlikely to bear any relation to how you have done. Indeed, most people are the worst judges of how they actually did.