People use this particular proverb to analyze by the looks how it agree on their personalities, but it is not always viable. Once I saw an indigent on the street, this person were considerable filthy, suddenly a friend of my have a talk with him and we discovered he studied in the university but he was a victim of circumstance that brought him into the ruin. Despite this, he was a person who can keep a large argue about politics.
On the other hand, there is people who looks like a serious person but on the inside have a really dirty history. One example is the policies of today, they are presumed to be legitimated and not rascal, rather the majority of them has done illegal things related to money.
People is assuming others' stories and your principles only by looking at you and that is impossible (although you were a psychic), you can not restrict a person to a facade and not to get know their stories.