If you read me really, really closely (I put those double adverbs in there just for you!), and especially the chapter in Sin & Syntax on Interjections, (and maybe also Pronouns and Voice), you'll see that what I most want is for writers to understand the different valences of words, and to be able to quickly spot "trash" words so that we can take them out when we want lean and supple and startling sentences but leave them in (or even add them) when we want a looser style, or more informality, or when we want to convey something specific about a character.

I love beautiful, powerful, elegant prose, but I also love the colloquial.

The point is to make sure  all words we use suit our purposes, to have a nuanced understanding of how to use words to our advantage.

So the idea of leaving words a dad might actually use in a letter to his son is completely fine, as long as they add to the believability and power of the prose. That thing about power is key. These words can often add something in conversation but detract in written language, so the true question is "Is this what I want to say and the very best way to say it"? As authors we have to think not just about our own "expression" but also about how words will register with a reader. We try to control how they register with literary style.

Have I proven why I'm fascinated with such questions? There is no easy answer, and I will allow myself to resort to an editor's cliché: "the proof is in the pudding."

—Constance Hale