If your friend was talking about you behind your back what would you do?

First of all you need to hear him or her yourself to make sure. Then if your friend was talking about you behind your back you should go up to him or her the next day and ask him or her if he or she was and then if they say no say well i guess we aren't friends anymore. When they ask why say because you just lied to me and i know because i heard you talking about me. Then walk off and go about your business.