I am going to apply for a firm where a mutual friend is the CEO, it is a startup though. I once had an interview with a mutual friend and i behaved casually but not too much formally but i did not get the job there. Do you have to be professional around friends in an interview?
Prepare for the interview as you would any other. Behave the same way you would as if your friend was not there, because he still may not be. You can discuss with him if you want, but ideally, leave it as is. You still want to wow the other members of the interview panel. You want to give them the feeling that they're hiring you because you're the best candidate, not because you're the bosses friend. The boss may even come away more impressed with you. If you're hired, you have to work with these people after the fact, and you need to come in with a good impression, not that you got a hookup.
I've had an interview where I knew every person interviewing me (previous job I had left 2 years prior), and one was a close friend. I treated it just like any interview, and while they already knew my work from previous experience, they still have a business to run. They appreciated that I took it seriously, and that gave me an edge. Are you the right fit for them at the right time? If you're not serious about their position, they may still like you, just not like you for their role.