Daring Fireball author John Gruber, in an extensive discussion with Shawn Blanc, outlines the ideal email interview:

You need to be really well-versed, as the interviewer, regarding the work of your interviewee. You must be prepared up-front with questions that you’d like to learn the answers to, but at the same time, I think you have to be ready to let the interview veer into unexpected territory. You want a plan, but you also want to be able to wing it as it goes. The mistake I see in most interviews conducted over email is that the interviewer simply emails the subject a single list of questions, all at once. Just a two step process: “Here are my questions,” then, “OK, here are my answers.” There’s no room in a simple process like that to ask follow-ups, or to delve into details or pursue interesting but unanticipated digressions.

I’ve often found “unanticipated digressions” or the occasional off-topic question, although sometimes surprising for those I’ve interviewed in person, is a great way to uncover or learn something that may not have otherwise come to light.

While such a process isn’t as straightforward via email, it is certainly possible, as this interview shows.