There is nothing particularly odd about the investigation of Humans. Meeting a person you have never seen before is almost to pick out a new book from the book store. The thing is to start reading, carefully. And by carefully I mean that you´d better read the lines and the spaces in between the lines as well. Reading the spaces means that there are no empty spaces, even if the person in question tries to hide some important information, acting negligent or ignorant. All of us adults are acting trying to hide things. No one invites others to read all lines and all so called empty spaces in her or his book of life. So, as an investigator you have to listen carefully and to play around with your questions in order to find if not a front door so a back door or a more or less hidden window.
Some of my younger colleagues have learned that the best technique is to follow a pre-designed interview questionnaire that is called standardized (because it is developed and proven to follow a sort of exactness and precision). But this is not as good as it sounds as such interviews will be conducted in a stiff, insensitive way. Each person you meet could be regarded as a self-built, very special “house”, and there is no sense trying to become invited into those houses using exactly the same tools in the same spots. You have to become more creative than so, trying different keys in different orders to find your way in.
And how about test instruments? Are they not even better and more objective?
Well, I would generally say no. First: psychological tests are quite rough instruments. They are not that very precise. Second: as tests are constructed by Humans they are in fact not that very objective. They just seem to be objective as it is easy to present the results as a diagram or just numbers on a scale.
Good investigations require good meetings eye to eye and good opening tools that could, and should, be used in flexible ways.