Be engaged, build rapport, engender trust, find a suitable environment, let them know all answers are “safe” (anonymous), they won’t be judged, and their opinions aren’t right or wrong... they are important when personal. Ask for examples and analogies, definitions, etc. Let them talk. Make them feel comfortable. Good interpersonal skills. Talk a little about yourself (i.e., humanize yourself). Adhere to common international ethics of research. Stuff like that. It takes practice. All respondents are different, so you have to adjust accordingly and don’t expect it to work all the time. Some people won’t synch no matter what. That’s OK. You or they shouldn’t feel bad. Just is what it is. Hope this helps. Most of all, have fun/enjoy. It tends to bring out candid and useful responses when you are enjoying your work, engaged, and respectfully passionate about their opinions.
I second David's response. Polite, professional and carefully thought-through and concise initial engagement. Explain research. Be clear about how data is processed/ anonymity/ how it is being recorded etc. Physical interview better than email/ phone if looking to elicit more thoughtful and thorough responses in my experience. Be patient and respectful at all times