I ran across this article and thought you guys might enjoy it. It boils down to the fact that everyone in everyday life uses body language to express what they’re thinking, feeling, etc. The same should hold true for the characters you create. You want them to be as alive as possible. A humanized character can have a reader rooting for villains even as they cheer for the heroes.
These are best used as dialogue tags as the author of the article says. This way you don’t have to use “he said” or “she said” all the time. It’s alright every now and again, but you don’t want to clutter up the pages with it. By using actions such as body language, you solve both the problem of flat characters but also making the dialogue more natural at the same time.
It’s a win-win.