Don't talk crap about someone if you can't back it up, simple as that. Because when the other person confronts you, you will be put on the spot and you can't help it but mumble. Which BTW is very pathetic. So if you ever choose to express a way you feel towards an specific individual in a negative way then do it! Start venting all you want but have some guts and the balls to actually confront him/her when the actual time comes. Until that time where you become courageous than please bite your tongue.