what to tell interviewees

Honest, you have nothing to be worried about. Most people are happy to talk about whatever it is you're asking them about. Even if you have no professional questions or anything of the sort.

For example, if I knew nothing about sword fighting, I wouldn't evne know enough to ask specific questions...to I might contact John Clements, at the ARMA web-site and say "Hey. I know NOTHING about sword fighting...but, this is what I think more or less happens in my novel. Am I on the level? What do I need to know?"

One thing I've heard remarked is that people who know their stuff (specifically, I'm thinking of John) get frustrated when they try to teach people, who are just interested in having their pre-conceived notions validated.

So be polite, be interested, and listen. Everything else really takes cares of itself.

Research

You'll probably never have a problem. Most people love to talk about their jobs and their hobbies. Give them a chance, and you'll get more info than you can ever use.

My only suggestions would be to first see how much research you can do before actually talking to an expert so you're well prepared. There's no point in bugging an expert about something you can find in two minutes on Google.

Second, realize their time is as valuable as your own, so don't go on all day. Know when to let them go.

Do this, and almost anyone will be happy to talk to you.

Plus, if you do your research and THEN the interview, you can have more of a fun and interesting conversation with someone very smart, rather than an interview where you're just trying to write down an info dump. It's a lot more fun, and you might find yourself with a friend on the other end of it.