Last summer, a friend who had read my novel Painting The Invisible Man offered this compliment: "I enjoy a writer who knows her way around a thesaurus."
Admittedly, at first, I prickled. And although I tried, I could not rein in my response. "I do in actuality have a very good command of language."
"I didn't mean that as an insult," she replied with just enough of a hurt-filled tone, I felt an immediate need to do penance.
Truth be told, I do rely on a thesaurus...and I proudly own a superb Thomas Y. Crowell Company edition of Roget's International Thesaurus. I use it as a trigger when I am trying to recall a word I know conveys just the right connotation. For example, I may want to describe the rather staid Englishman's reaction, not as prudish, but....(here's where I may turn to Monsieur Roget, for I know the better word exists, but at the moment it refuses to break through my middle-aged brain)...Ah, yes! The word I seek is not 'prudish,' but rather 'priggish'. A much better choice of word for a male character.
But, the use of a thesaurus merely to impress will more than likely render language that sounds pretentious or just plain wrong.
Use your thesaurus wisely and your writing will have substance and style.