Superb interviewing is an art to be practiced. When it's done well writers can get telling little details that can be dropped into their writing like the perfect spice to highlight the best tastes of the meal. That said, there are some tricks that writers can use to move them along in the right direction.
A colleague at the St. Paul Pioneer Press, where I worked as a reporter a while ago, suggests that interviewers notice details about their subjects, then ask questions. For example, you may notice that the medical examiner you're interviewing wears a watch on each wrist. Two watches? Why? You may learn the doc travels a lot and can't keep local time zones straight and wants to always call home before his daughter goes to bed. Or, the doc may be nuts about being on time and figures two watches will double his chance of punctuality. Or, one may have been a gift from someone important to him but it loses minutes and he relies on the other watch to tell accurate time. In any of those cases, you've got a personality quirk that can help you define the medical examiner in your mystery.
Then you've got to figure the best way to record those answers. Some writers use digital recorders, often built into their phones, while others go the good old fashioned paper and pen route. I think there are advantages and disadvantages to both.
When using a digital recorder writers can forget about taking actual notes and hone in on the subject instead, confident that the device will do all the work. That can be very effective if all goes as planned -- which it never does. I've done that a time or two, only to learn the recorder didn't work, or couldn't pick up the voices or got accidentally deleted. Then I don't have a scrap of reminder to tell me what the guy said.
With paper and pen writers don't have to worry about technical malfunction, but unless the writer takes shorthand some words are going to be lost. Besides, paper doesn't capture inflection or weird little turns of phrase that, again, can be descriptive details for a character. Some subjects get nervous when they see a pen scribbling across a page but seem to forget when a recorder is capturing their every ummm.
Myself, I like to use both. Paper is a good backup to the recorder, and vice versa. But to be honest, I try not to refer exactly to the notes/recording after the interview, although I tuck both away in my files in case I change my mind. Rather, I like to just let the subject's words marinate in my head for a while before I write. That way, the most important details seem to bubble up, while I forget the stuff that would clutter up my pages with non-telling detail. But that's just me. What method do you use?