<p>do i call my interviewer by their first name or by Mr....?</p>
<p>Mr/Mrs unless asked to do otherwise.</p>
<p>I'm an alumni interviewer and I concur with SP. In life, not just in an interview setting, I would recommend calling adults Mr/Mrs/Ms until told to do otherwise. If they want you to call them by their first name, they'll tell you right off the bat. If you call them by their first name and they'd prefer Mr/Mrs/Ms, it's really hard to say that's what you want when someone has taken the informal route and then it just grates on you every time you are called by your first name. This is a real pet peeve of mine. I'll get off my soapbox now.</p>