As much as the "experts" claim that handwriting analysis is not an exact science (and is there such a thing as an exact science? Apart from mathematics, that is...), it is undeniable that the way people write, what they write about and the language they use tells us quite a bit about the person.
I only started looking into it because it struck me that two of the most narcissistic people I knew, though quite different from each other, both write in all caps when sending a card. I don't think this applies to all people who write in all caps, sometimes all caps can have a clear and pretty effect; what I'm talking about here are the ones that make their "get well" cards look like kidnapper's ransom notes. Also like kidnapper's ransom notes, the way they write is bullying.
Interestingly, both these people also send typed personal letters (I guess it takes too long to write in all caps) which I think is odd. If you're going to do that, why not send an email instead?
Here are some interpretations of why people write in all caps which, at least in the cases of the two people I know, are spot on. (from http://www.writechoice.co.in/writechoice/index.php/2011/08/all-caps-handwriting-why-some-people-write-in-block-letters-and-what-it-means/)
"these people do not like to disclose anything about themselves to others. They are uncomfortable talking about their personal lives — at least in interpersonal relationships. They do not want others to know about themselves."
Usually, all-caps handwriting lacks connection between letters, which indicates that the writer does not like to relate to people at a personal level. Also, such writers are quite egotistic. They hold strong opinions and it’s very tough to get them to see another point of view. They emphasise a lot on their day-to-day lives, rather than spending some time planning their future.
Another article on this I found on http://handwritinganalysis-1.blogspot.co.uk/2009/03/when-people-write-in-all-capitals.html
the comments were particularly interesting (a lot of examples of "the lady doth protest too much"). One in particular caught my attention:
"He looked like a really well put together kind of person. dressed nice and very polite.""His words were very carefully chosen." This is also true of the two Ns I know. I think the dressing well and being polite (and charming) is their decoy. In reality, they are bullies, but if they showed their true colours straight away, nobody would go anywhere near them, would they?