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".
I USED TO DO THAT ALL THE TIME. And it really embarrasses me now; I've actually gotten to a point where I hate adverbs in general and I try not to use them at all if I can help it. I see adverbs now as a form of telling rather than showing, and I urge all writers to limit (or eliminate) their use of adverbs from here on out. Check it out:
"To hell with this," Mike said fiercely.
Just no. Don't explain to me a tone of voice that I can glean for myself through the words being spoken. Take out the 'fiercely' and see for yourself how much more elegant the sentence is, and how pointless (and ugly) the adverb was.
Btw this is also a great - and related - rule, again from Leonard:
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 and go to the dictionary.
It is SO TEMPTING to do this (and I still do it a lot), because you think you're using said (or says) too often, but no one notices said or says, whereas other verbs stick out like sore thumbs. Again, you are telling your reader something that should be implicit in the dialogue, that you should be showing in another way. [I give a pass for whispering and murmuring though, because they are essential in porn.]
And while we're talking about bad habits that writers should watch out for, let me reiterate some advice I've seen floating around in fandom forever. Do not call your characters "the other man" or "the younger man" or "the taller man" or "the whatever-bizarre-descriptor-you-can-thin
So hey, this is fun right? Me doling out totally unqualified/unsolicited writing advice. Let's get a discussion going. Tell me about some of your pet hates, tell me about all those bad habits you'd like to see less of in fic.
Let's help each other be better writers!