We have all watched a cop show at one point in our lives. If you haven't then what planet have you been living on? Just kidding. My point is that we all know about how they bring them in and sit them in the chair. They make small talk to get them comfortable or no talk at all to freak them out. They start off with the easy questions before they drop the bombshells on them and try to get the confession or the evidence that they need. Well, preparing to interview a relative is something similar.
See every family has someone who loves to talk. In my family that would be me :). But I'm not the only one. There are some people in my family that if you walked in and asked "Tell me about you parents?" you would get two hours worth of stories and nothing that helps your cause. They would tell you about their parents marriage and family trips and moms temper and dad's love of fishing. They would share about anniversaries and conversations and you would leave knowing them better as a person but with no real facts to follow up on. That is why you have to go in with a plan.
You have to know what you want to get out of them. You might be able to get the whole confession out of them or just evidence to lead you to the real culprit. What I mean by that is that this person might not be the one with the most family secrets but they might know who to ask without knowing it. However, if you just go in with simple open ended questions then you will never know.
There are dozens of websites out there that have list of questions that are good for interviewing relatives and I would recommend them all. They all have questions that overlap and those that are different but at the end of the day they all have questions that can help you. You more or less won't use ever question. Pick and choose which ones will work the best for you and what you want to figure out. I recommend questions about maiden names, full names, places of birth, anything with a date, children's names, and any moves to different states. Those will be life savers once your really get started.
You also what to make sure you have enough time to get all the information. Don't go when you have to pick up your kids in half an hour or when your on your lunch break. Conduct your interviews when you have time to spare. A Saturday when the kids are gone with friends or away at camp. Your day off when everyone else is busy. And please learn from my mistake. Holidays and family get togethers are not the best place either. With holidays everyone has their minds elsewhere. They are thinking about the food and gifts and enjoying the party or even football. They can't give you the focus that you will need. And with family get togethers it's to much. You know that saying "to many cooks spoil the meal"? Same thing. One person will start to answer your question and then someone else will jump in with their version of it and by the end you have one relative with six birth years and four different names. It's better to speak to each person individually that way even if you still end up with multiple information you know that it wasn't because they all thought they knew better but because that is what they were all told.
Interviewing can be tricky and there really is no right or wrong to it. When it is all said and done it is just about getting the information you need. If you accomplished that then you are way ahead of the game.