Some of the best advice I got when starting out as a writer was avoid the temptation to show the person you're interviewing how smart you are. For example, I used to write magazine articles about science, so I was interviewing researchers who I really respected and thought were doing fascinating work. I have a Master's degree in science, so it was hard to resist the temptation to show this amazing scientist that I knew my stuff, and it was also hard to avoid lapsing into jargon when we were talking about a field I knew well.
But if you fall into either of those traps, you're not going to get the quotes you need for your article. You need the person you are interviewing to explain what they know, in their own words; if you start showing off, you're going to put words in their mouth. Even worse, they'll realize you know stuff, and they won't tell you the important parts, leaving you to write about it without quotes.
Avoid Biased Questions
A related quick and dirty tip is to avoid asking questions that will put the person on the defensive. "Did you mutilate the cute snail after using it for your own purposes?" is not going to get you the same answer as "What happens to the snails after the experiments are finished?"
Avoid Leading Questions
Similarly, you don't want to ask leading questions. A leading question leads the person to the answer you want. For example, "Isn't that police inspector the nicest, cutest woman you've ever met?" is a leading question. After a question like that, your interviewee isn't likely to say, "No, the police inspector is a horrid wench," which would actually be a fun quote to get.
Sound Dumb
What this ends up meaning, is that through most of the interview, you often end up sounding like a dull, rather dense, but perhaps inquisitive lump of coal, interjecting statements such as "Can you tell me more about the police inspector?" or "What are the implications of your snail research?"
Be Smart
Of course, this does not mean you shouldn't do background research or have interview questions prepared. You need to know where your story is going so you can probe in the right direction. You need to know the person's background and as much as you can about the topic of the interview. Although your job is to mainly get your interviewees talking and then listen, you need to know when they say something of questionable accuracy, or just misspeak, or if they are getting off track. There might be something interesting that they have forgotten to mention, such as an opposing theory. For example, "OK, Dr. Jones, but what about the people who think that your data is merely the result of tainted water?" It's also helpful to understand the process behind their industry. For example, you might need to ask, "Well, Dr. Lopez, how can you get your drug to market by fall of 2008 when the typical trial takes four years?"
Follow Up
And finally, I'm going to share with you my secret weapons for interviewing and getting story ideas. My last interview question is always, "Is there anything I haven't asked you that you think is important or worth talking about?" I always asked that question no matter how much preparation I did for an interview, and about a third of the time it got me something I hadn't anticipated.
If you end up chatting with the interviewee a bit after the interview, another great way to follow up, especially if you're a freelance writer and have to come up with story ideas, is to ask the person what else is going on in their field that they think would make a good article and who other good people would be to interview . This can often lead you to other interesting stories you didn't know about.