know your subject (to a point)
How to prepare: research
Every interview requires some prep. Knowing your subject well (as well as can be expected) before you sit down with them helps you not only arrive prepared with productive questions but gives you greater control over the interview. Plus, as mentioned before, showing that you’ve done your homework is the quickest way to earn the respect of your interviewee. Showing that you haven’t is the quickest way to lose it.
Sources for backgrounding info
Social media: Nearly everyone has some social-media presence now, so look up whatever might be publicly available — Facebook, Twitter, Instagram, etc. For professional purposes, don’t skip LinkedIn, where more work-related info might be. (Remember, you’re looking for basic background information about the person; you’re not cyberstalking them. Do not bring up in the interview: “Hey, looks like you had a wild party last weekend!”) (Also, this is a good exercise for you: someday a potential employer will be doing this to your social-media feeds. Think about what they’d find! Act now to edit!)
Company info: Companies doing business seek to (or are legally mandated to) provide information about themselves publicly. Look it up — the company’s web site, trade magazines that might report on the company, annual reports (this and other incorporation info can be searched in Lexis Nexis, available through the library), etc.
News media: Search for the person and his/her company in the news media, via Google News (for more recent, broader results) and LexisNexis (for more thorough, historical results).
Colleagues: Talk to colleagues and friends, if available. It’s perfectly routine for you to write/call someone and say, “I’ll be interviewing Jane Janey about her position at Megacorp, and I’m trying to understand the organization best before we chat. What can you tell me about Jane and where she fits into the organization?”
Yourself: You likely already know something, if not a lot, about your subject and/or their field. Perhaps you're interviewing a family member or acquaintance; think about what you've learned about them in the context of their work and their (mediated) communication with you.
What if there's not much to find?
Well, good — that means your project will contribute something in an area where existing archives and the Internet are weak. The goal is to get as much of an idea about who your subject is in order to make beginning and facilitating the conversation easier for you. If you can't assemble much of a profile ahead of time, then you'll simply need to focus on obtaining that information when structuring the interview.