These add to character as well. I have a notebook into which I jot such observations. The best part is, in fiction, you are God. You don't have to be right in real life about your observations. In fact, in real life, I avoid drawing hard, fast conclusions about people and try to take them at face value.
Real people are too complicated to be read like a book.
But in fiction, I define the truth. If you can get a detail that seems to indicate a particular state of mind, you can lead the reader into believing it. I could write the same book for years, something the romance market doesn't generally allow. The pace of romance publishing is hectic, while I tend to overwhelm myself with a thousand details that, to address them all, would take a decade. Happily, the editorial process allows for some fine tuning and polishing with another pair of fresh eyes on the manuscript.