Everyone's two faced to a certain extent. I can be two faced, normally because I am sparing someones feelings/dont want the hassle but then still need to vent in another direction. It's the person that lets your little vent slip thats causing the trouble.
Then if you say nothing to anyone, to avoid this and develop no public opinion, you get picked on for being a fence sitter. Im not a fan of telling it like it is all the time, wears you down a bit.