Podcast guest question

A friend of mine and I do a weekly podcast on books and reading called The Library Police. We recently had an author ask if they could be a guest on our podcast, which we're excited about. The problem is, they have to phone in. We do a local podcast, using two mikes fed into a mixer, which outputs to a laptop running Audacity. What's the best way to set up a phone guest (like with Rabbit and Demiurge)? I'm thinking Skype, but I'm not too familiar with how that would work. We would need to listen and interact with the guest, and record their voice as a separate track.

Since you have a mixer, Gaald's article on how the GWJCC is recorded might prove useful for you. The one other thing I would suggest is to have your guest record his local mike using Audacity - that way if your skype call drops or turns out to be very poor quality, you can always mix the tracks in post-production.