Audacity is a free, easy-to-use, multi-track audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems. The interface is translated into many languages. Audacity is free software, developed by a group of volunteers and distributed under the GNU General Public License (GPL). FREE DOWNLOAD
If you are interviewing someone remotely then Skype is your friend. You’ll need an add-on to record the interview, and for that I recommend Total Recorder (for PC) or Call Recorder for Skype (for Mac).