How come a person who may be generally pleasant and amusing can at times be a miserable grouch? Is it a facade they put on or is it a facet of their personality? For that matter, what's the difference between the two?
A facet is an aspect of a person. We are all multifaceted. Sometimes we are contemplative, other times we are passionately determined, and other times we are completely apathetic. Nobody feels one way all the time.
A facade is an illusion perpetuated by a person. Sometimes we like to be somebody or something that we aren't. We do it for a host of reasons and to varying degrees, but it is a way to present something that is not our true selves.
And that really is the difference between the two: A facet is part of who we truly are, but a facade is just an illusion - it is not something that comes naturally, but something that is actively done.
When your characters act "out of character", ask yourself if it is a facade or a facet (or if it's just bad writing). Characters can do either of them, but there is always a reason for it, even if they don't know why.