Some fish turn to be dangerous, but in fact they are not! now my point you cant act beyond something or someone until you know them very well.
some times you could have bad impression for someone, and later you discover all your ideas about this person is wrong! while you may have been mistaken about other people where you give them all your trust but they don't really deserve it.
you should know exactly who is the person you are dealing with before it is too late!