ALL of this. Encourage people to try new words, to mess them up, to experiment with vocabulary, to learn complicated adjectives and verbs and nouns, because words are fun.
Also, don’t be a jerk.
I still mispronounce run-of-the-mill words (ex: subtle, quinoa) that I have read for fifteen years but never heard out loud. My first couple years of junior high and high school were full of moments like that, where my old and oversized vocabulary smacked into my nascent social life.
This is the kind of thing that happens when books are your childhood friends.