Interviews

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First rule: If you talk more than the interviewees, you are doing it all wrong! Don't try to fill silent moments, don't lead the discuss too much!
Interviews are definitely a must-do in your process, you want to design for the people, so go ask them what they want. And you can do that in 4 main different ways.
Intercept interviews are the most quantitative ones. Go to public places, meet a lot of people and ask them questions for 1-5mins, not more. You'll probably get a lot of refusal to even talk but it's part of the game! Maybe you can find a way to attract the people to you, with some goodies... You should also make it obvious that you are not trying to sell them stuff. Queues can be a good place for this kind of interviews, people will get some distraction while waiting.
Scheduled interviews are more qualitative, they require more time and planning. Doing this kind of interview at the interviewee's working place, home (depending on what you are designing) can be a good idea, they might feel more comfortable than in an unknown office. Don't try to dress up "business style" if it's not how interviewees are dressed, try to also "fit" in their world with your clothes. This interview can last around 30 minutes and recording it in some way can definitely help you to catch everything later on. At least 10 interviews should be conducted, 30 is a good number and 50 is too much (but obviously all this depends on the size of your team and what you need to learn).
Group interviews are a discussion, where everyone should be able to talk. It can require you to facilitate the conversation. You might not go as deep as during a one person interview, but this method can create debate, comparisons and finaly gives a lot of insights. A group of 5 people can be enough, to avoid having some people never talking and to keep things on the right direction. Depending on what you are working on, you can try to create groups of completely different people or groups of similar people, the results will be probably not the same and this is a good way to get a wider range of insights. Depending on how long you decide it to last, the location... you can decide to pay the participants.
Expert interviews are about getting detailed information on the topic you are working with. Meeting an expert can be much faster than an internet research, you should consider having this kind of interviews all along your process.

You shouldn't do interviews without being prepared. Have a draft of questions, even try them on one person to be sure you can get something out of it. Bring competitors products to help interviewees talk. When emailing or having a first contact with the person you want to interview, you should tell them that you need "their expertise", this will make them feel important!
Make clear
with them how long it will take and that they are totally free to not answer. Try to push them talking about the problems, not only what is good: they might feel like they have to say good things to not be seen as hurtful. Always ask about the full experience, don't focus only on the part that you are interested in and make them talk about the sequence: "Take me through what you did to...".
Being two can be a good solution, one dealing with the interview and the other dealing with notes, recording, all the "backstage" tasks: there is nothing worst than answering questions to someone having the nose into his notebook! You should adapt to your interviewee, with your clothing, like said before but also with your language: if they use the word "cool", don't change it to another word.
Try to find "lead users", people who were the first and have been using similar products for long. Focus also on "extreme users", the ones who use the products in unusual ways (disabled people, the ones using the products in extreme situations...).
A good way to find deeper needs is to use the "Why?" method. Start with a quite open question and keep asking "why" after the answer to dig more and more.
Other interviews can also be conducted during other methods like shadowing.