I've been interviewed quite a few times, and yes there is definitely an art to it. The secret is *not* to interview the person, but to talk / chat to them whilst filming. That might sound the same, but the key is to make the interviewee relax and talk naturally, otherwise it sounds terribly stilted and fake. As soon as you point a camera at someone, they normally totally lose their ability to talk in a natural and relaxed manner. Also, never use the word "interview".

These are just my opinions on the best techniques, but don't get the interviewee to look into the camera - they're not a presenter. Put the camera on a tripod, start recording, switch off the little record LED (in the menu) so the interviewee doesn't have a constant reminder that they're being recorded, and then position yourself to the right / left of the camera and just start chatting to them. If you absolutely must be behind the camera, get a second person / friend to do the chatting. Use any strategy to get them to relax, and then introduce the questions you really want to ask into the conversation. Remember you can always edit out your own voice. It might take 10 minutes before they really start to forget they're being filmed, and that's when you'll get the best material - worth losing 10 mins of tape over. It's also better to interview them later in the day when all the stress of the wedding is behind them, and they've started to relax quite a lot. Of course, you don't want them drunk, so chose your moment carefully. Actually, you could interview them before the actual ceremony if you can somehow arrange it, to get that "How do you feel right now?" question in.