I have an interview coming up. I have an MS and one of my friends from the program at the same time as me recently got his PhD. He is in what would be a managerial role were I hired. We knew each other in a not totally heartless capacity, but we weren't best friends. Should I refer to him as "Dr. _" or by his first name, as we previously knew each other?
This will depend on the part of the world you are in and the preferences of your friend. Based on your profile, I'm inferring that you're in the United States. Generally, in the US, people don't use the title "Dr." unless they are referring to a medical doctor. If you were in Germany, on the other hand, people tend to get offended if you omit a title. Unless your friend has expressed a preference that he be referred to with a title (which would generally be considered a bit pompous in the US but hardly unheard of), you would generally just use his name.
Hopefully, of course, someone in the interview will tell you the proper form of address by using his name first either by introducing you two (i.e. "And you know Dr. Smith/ Joe, I believe") if he's going to be in the interview or by referring to him in conversation. If they do, it would make sense to follow their lead.
My approach would be:
Justin's right - the US is pretty casual - we mostly go by first names in most office settings, but once and while, you'll run into someone who's a bit more formal, and then the informality can be offensive. Asking is rarely, if ever, offensive, especially when it's done confidently.
After you have an answer from him, you'll probably feel 20 times more confident, which should help the interview process considerably.