Handwriting is a form of expressive movement – it is our movement frozen on paper.
There are lots of ways that we observe how people express themselves. When we talk with someone on the telephone we not only listen to what the person is saying, we listen to how they are saying it: their tone, speed, inflection, emphasis, hesitations, loudness, and many other things. Some studies have shown that over 80% of verbal communication is how something is said, and so we have learned to listen to these signals. When we are talking with someone in person, again we not only listen to what they are saying, but how they are saying it. Additionally, we look at their body language. We notice how close they stand, if their arms are crossed, if their eyes are wandering, and so on.