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 LeonardThese are rules I've picked up along the way to help me remain invisible when I'm writing a book, to help me show rather than tell what's taking place in the story.
Elmore Leonard