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.
Elmore LeonardTo me, writing is the most fun. It's not always fun, but finally when you make it come out the way you want, it's then you can say, 'It's fun, boy.'
Elmore Leonard