I work with someone who is a backstabber to all that know them. When I became new in this position one of my friends warned me about them and gave me advise on who to trust. Boy, was I grateful for her advise. She was right on the money.
Since I have learned what this person is like I now keep track of any work that I need their approval on and keep a little notebook with this information and go back and look at the information if I did not get a response for it. I keep track of emails and save them along with decisions they make in case it would come back to haunt me.
One time I caught them sending a nasty email to a director about myself and a coworker. They tried to deny it; but it came up on that directors email when we were doing a conference call and he had his email up and everyone could see it. I now just choose to ignore them except for issues I am forced to deal with. I will not talk to them about personal stuff or ask them questions because I know they use those questions against you by going to the boss and complaining even if it is a valid question.
It drives me crazy to work with people like this. I have at least found co-workers that I know are trustworthy and ditch the other two that I know are nothing but trouble.