The best way to approach someone you want to interview is by personal contact, rather than by letter, and often the initial contact will be by telephone. This gives you an opportunity to introduce yourself, explain your project and outline the sort of topics you might cover in your conversation. So you sometimes have to do a bit of persuading. When you speak to them get some background information and decide where the interview should take the place. The person's own home is by far the best as they will be much more relaxed.
Privacy encourages an atmosphere of trust and honesty.
A third person present, even a close partner, can inhibit and influence free discussion. They will be as nervous and apprehensive as you are, so it is essential to be cordial and patient. If you can, switch radios and televisions, which can sometimes make it difficult to hear what someone is saying. With a hand-held microphone place it as near as possible but not on the same surface as the recorder, nor on a hard surface which gives poor sound quality. Generally, the closer the microphone the better the results.