A thesaurus can be a marvelous animal to help find an appropriate word for your writing,
the perfect little word to contain everything you’re trying to say elegantly and so simply your readers will understand immediately the mot juste.
The Economist’s vocab survey of 2013 says most native English speakers have a vocabulary of 20,000–35,000 words which seems a lot to choose from without needing to reach for a thesaurus. Readers (particularly fiction readers) will have a much wider vocabulary than non-readers, see below. If you want to write with an authentic voice, the word in your head is usually the best.
Writers, I know, often have an inbuilt thesaurus, but this is a very personal thing. Innocuous word such as “member” or “lay” will flash different alternatives in the lexicon of a parliamentary writer and a writer of bodice-rippers, and I mean flash there in the sense of display, not flaunt, show off, or expose one’s genitals.
For most of us, good writing is about taking words out. Edit until a thought is clear on the page, and perhaps then a thesaurus might help with precision.
Tame your thesaurus and use him to gobble up your prose and spit only the good bits out, perfectly regurgitated. Never ride your thesaurus at a gallop. Walk him through your writing in reverse, to condense long, complicated descriptions into clean, tight text.
You are not in a confrontational predicament on meeting a Thesaurus in the wild. You’re in the shit.