When I interview authors I usually send them a list of questions (via email), and then they email me back with the answers which I then format.
What Sarah said below is basically it. You can also send them a list of questions and ask them to choose a certain number to answer and send back to you.
What Sarah and Tony said is the way it has worked every time I've done an online interview.
Hi Joyce, speaking as an author - who has done some Q & A interviews - the question and answer interview really works well, but to make it work well get to know the author's works and genre, then add in a couple of questions that are more specific to their work. This will make your interview more interesting, for the readers and the author!
Children's author and illustrator