When someone writes totally in capital letters it’s because they don’t want who they truly are to be seen. They may well be totally unaware that that is why they are doing it, but that doesn’t change the fact that it is!
People who are just looking for clarity sometimes print. Engineers, accountants, architects are some of the employment groups who very often print because exactness and accuracy are so important in their line of work.
However, capital letters provide no further clarity than printing, so all caps is not done for clarity.
The least possible is revealed from writing in all capitals. Printing shows less than script, but when all the writing is in capitals, it hides even more.
Most handwritten ransom notes, for example, are written in all capitals!
You can get something from it however. For example, the slant will still tell you the emotional make up of the writer (the farther to the right, the more the emotional expression), and the heaviness of the writing will still tell you how deeply emotions affect this writer.
There are a few other indications that will still show through in all capital writing, however, the main thing to think about is that this person, knowingly or unknowlingly, is hiding themselves away. The question is “why?”