I tell my students the same thing. There are three grades of language to show character:

Good: Description. You can get sex, general age, health, and depending on your world information about class and so on from clothing. Grooming can sometimes reveal personality. But as everyone knows (I hope), appearances can be deceiving. Avoid phrases like "arrogant cheekbones" or "haughty lips."

Better: Dialog. Are they polite? Friendly? Outgoing? Shy? Sometimes you can learn a lot about a person by how they speak to a waiter or a car rental agent. But people lie, right? How revealing of one's true character are the words spoken on a first date versus after ten years of marriage?

Best: Action. Actions speak louder than words. It's not just a proverb, it should by tattooed on you somewhere if you want to write stories.

So I took an old favorite of mine, Rikki-Tikki-Tavi by Rudyard Kipling, and color-coded the verbs by character (Adobe reader to view the pdf file required). I skipped "to be" forms and saids and so on, but that's not such a big deal, as the story is loaded with descriptive active verbs. Rikki is in red, since his eyes are always glowing red when he's fighting the snakes. The cobras are green, and so on. There's a key. Skim it and see what you can learn about each character just from the colored verbs. Then go through and read the story (it's only a little over six thousand words) and see if the characters match their verbs. Brother Rudyard tells us that the motto of all the mongoose family is "run and find out." Is this borne out by the verbs used to describe Rikki? Look for the power and menace, and yes, fear of the mongoose in the snake verbs. And the rather aloof but kind verbs used for the family in the Segowlee bungalow.