There are a number of situations in which you might already know the hiring manager for a particular job. You might be a college student seeking an on-campus job with a department where you already know some of the staff or faculty members. You might be a new graduate interviewing at company where you interned for several months.
When I interviewed for my current job, I already knew almost all the people in my department because my husband had been working there for a year. One of the people interviewing me was a good friend. I had to strike the right balance between personal and professional to successfully handle the interview.
Whatever the case, here are some do's and don'ts for this kind of situation.
DO take the process seriously. Put together your application materials with as much care as any other job, and dress professionally for an interview just like you would to meet a stranger. You don't get a free pass on any part of the process, and acting like you do is not going to work to your advantage.
DON'T assume you have an advantage. Someone who has a good relationship with you may think you're a great person, but that has very little to do with whether you're capable of doing the job at hand. In fact, if they know you well, they may be more aware of your faults than a candidate they've never met before. Be prepared to make a strong case for yourself as it relates to the job you're interviewing for.
DO acknowledge that you know each other. You want to be professional, but there's no reason to be abnormally stiff and formal about the whole thing. Give them a firm handshake, but say, "It's good to see you again" and ask after their family members, mutual acquaintances, or past projects you worked on together. It's also OK to mention in the interview ways in which you believe your individual work styles mesh well together or specific past experiences with the person that led you to have an interest in working there.
DO focus on the importance of fit/match. One fear an interviewer who know you may have is that you will react badly if they go with another candidate. Show that you're mature enough to understand how the process works by proactively mentioning that you want to make sure the position is a good fit for you, and you understand that they need to find the "right" person for the job. This helps reframe the discussion from the awkward "I am choosing / not choosing you" to "We are both determining if this is a good match for this job."
DON'T take rejection personally. This framing I just described is important for you to keep in mind as well, particularly if you're not selected for the position. At the end of the day, the person you know has to choose the candidate they think will do best in this specific job, and that is unrelated to their personal feelings about you.
Have you ever been interviewed by someone you knew, or had to interview someone you knew? What was your experience?