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.