Then you already failed.
They know your background if your resume is accurate. And they know you don't have a lot of life experience. You're being hired as an intern. Keep that in mind.
All an intern amounts to from our perspective is an extended interview. We get a look at you and your ability for months rather than 30 mins. We could get a trained monkey to do the tasks you will be immediately assigned, however we don't because the intern might be good and turn into a firm leader in 10 years...
They are concerned mostly with "fit" at this point, this includes if they think your personality fits with firm culture, and if you'll be willing to eat the shit sandwiches you'll be fed. If it is a larger firm, you'll have more wiggle room here.
So, you NEED to be yourself. Do not "sell" yourself as something you think they want you to be, be who you are. I assume if you've gotten this far in accounting, you're aware of who you are and how you feel you fit in the profession. Go with that.
Good, so maybe you haven't failed.
Look you need to think beyond this internship. This internship is nothing but the first stepping stone to your CAREER in public accounting.
Think like this, live like this, speak like this, and be open about the fact you are interviewing them as well, because you want to know you can move upward in the place you decide to work.
Questions like "I know this is for an internship, but say I was a full time hire, what is my path to partner?"
"What are the expectations of someone in my position, and do the expectations of an intern differ than that of a full time staff hire?" (You want to emphasize that you don't want lowered expectations.)
What you want to tell them, in not so many words: I'll be your donkey you can ride into the ground, that will provide quality work and eat all the shit I have to, but expect me to be gunning for your job and painting my name on your door eventually.
They want hunger, fire, people that can see the macro and micro and communicate.
Don't use slang, but don't try and use vocab you don't master either. Err on the side of prudish, avoid politics or religion talk, and scrub the shit out of your facebook and twitter.