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”.
Here’s Philip Pullman’s response in full:
My main rule is to say no to things like this, which tempt me away from my proper work.