Here are two more of Elmore Leonard's recommendations for writing.
3. Never use a verb other than "said" to carry dialogue. The line of dialogue belongs to the character; the verb is the writer sticking his nose in. But said is far less intrusive than grumbled, gasped, cautioned, lied. I once noticed Mary McCarthy ending a line of dialogue with "she asseverated" and had to stop reading to get the dictionary. (Asservate means to declare positively)
It took me a while to learn this. And I'm sure I've made some verb errors since I did know it.
But that's pretty black and white. Leonard says "don't do it" then it's a pretty good idea to follow.
4. Never use an adverb to modify the verb "said" ...he admonished gravely. To use an adverb this way (or almost any way) is a mortal sin. The writer is now exposing himself in earnest using a word that distracts and can interrupt the rhythm of the exchange. I have a character in one of my books tell how she used to write historical romances "full of rape and adverbs."
They have a nasty way of sneaking into a sentence. Ex: "She reluctantly said. He quietly said. She very delicately mentioned." Eek.