Act now the way you want to feel about yourself later.
So go in to the interview with the enthusiasm that you got the job. Start your presentation thinking you will hear “BRAVO” at the end. Approach that cute person as if they already think you are the coolest person they ever met!!
It is all about taking away the fear of the unpredictible. Granted, it may not work out EXACTLY like you thought it would in your head, but at least you did it.