I think about this a lot when writing (and apparently I write a lot - just not fiction), so you might have heard this before.
I once read a quote by Stephen King, who said, "any word you have to hunt for in a thesaurus is the wrong word." I've always disagreed with this sentiment vehemently. (I'd be more inclined to forgive it for being taken out of context if the next sentence wasn't "there are no exceptions to this rule"). I suppose it's aimed at "try-hards" who want to spruce up their vocabulary by using words they're not very familiar with. (Is there a word for people who are pretentious about being unpretentious? Because there should be). But my mind works in such a manner that I'll often have a specific word in mind that I want to use, but for some reason, I can't remember exactly what it is.
It's like trying to look at a dim star in the night sky. If you look at it directly, it disappears. But look to the side, and there it is, in your peripheral vision (this is due to the placement and sensitivity of the rods and cones in your eyes). I have a vague sense of the word I'm looking for, and sometimes I stumble onto similar-sounding words that ultimately have different meanings. Using a thesaurus is the best way for me to save myself some agony and potential embarrassment, while preventing me from having to use a synonym with an altogether different connotation, that sabotages the intended meaning of my sentence.
Maybe my vocabulary would be better if I read more (but who has the time? :p), but I never use the thesaurus to find words I don't already know. That's just not it's function in my mind. Although I do sometimes come to the conclusion that the word I'm looking for may not actually exist. Yet.