At which point he clenched his jaw

What is it with jaws? I don't mean jaw-jaw is better than war-war. I mean with people trying to write fiction, and, sometimes, being published. There are always problems for new writers in trying to use dialogue - two people having a conversation, like playing tennis. After you say he said/she said a couple of times you feel nervous so in come the he stammered, he averred, he blurted, he agreed, he smiled, he hissed, she snapped, she shrugged, she nodded... and if you go back to Victorian times, he ejaculated. The latter is not recommended for modern usage in dialogue unless, of course, someone did. Most of the others should be avoided too. He said is usually enough, and if two people are in dialogue it is easy enough to work out which one is which, with the occasional "Tracey said" if you feel the reader is getting lost. And, besides, the words used should carry the sense. If I were to write "I hope you die" the sense should be clear, and you do not need to say ..."I hope you die," he shouted angrily. For an object lesson in how to write bad dialogue, try any Harry Potter book at random. OK, they sold a copy or two but I'm talking art here not dollars.
But back to jaws. Now, go to the mirror. Clench your jaw. Then try firming your jaw line. Then you will probably think, "I am unable to do this. I have no idea how to achieve a firm jaw line." (I'm assuming you don't normally stand with your jaw dropped open.) So why, why, do writers wishing to write fiction have an obsession with this otherwise unheralded part of one's anatomy. I have read it so many times, and, I confess, read it in two of my late and much loved cousin's published novels. I wish I'd seen the manuscripts before publication. I'm currently reading it, in a book not being published by Five Leaves. It is jaw-dropping.