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 LeonardI do have fun writing, and a long time ago, I told myself, 'You got to have fun at this, or it'll drive you nuts.'
Elmore Leonard