What is the best way to record an audio conversation made in Skype?
I suggest you use Ecamm Call Recorder. It's a plugin for Skype that will record both sides of the conversation into separate tracks. If you need high quality audio, you could have your interviewee record their own audio locally with QuickTime and send you the audio file. We use a combination of these two to create the Ask Different Podcast - individual QuickTime recordings for quality, and a Skype recording that we use to sync up the conversation.