When is it appropriate to use a synonym that you find in a thesaurus?

When you understand the connotation of the Word compared to the original Word.

Use a synonym when the Word you use is being over-used. For example, brochures that I used to compose needed the Word ensure. I myself tired of that Word, so I used insure for a few pages. Then I used "make certain". Don't bore your reader.

Like the novels that are written nowadays. In one book in one paragraph, I counted 9 had's in about 5 sentences. Example: why did so-and-so "had had" a coke in the morning when he "had" arisen from bed, where he "had had" a cigarette. Then he "had" taken a shower and "had" shaved.

All those had's show the ignorance and stupidity of the editors, because I know of a surety that Lee Child does not write like that, much less speak like that. Neither does John Grisham or Terri Persons. OK. Bad analogy. Let me get back on track; but you know what I mean, right?

How about bottles and change it to glass containers? It's appropriate to use synonyms when you can, when you should, and when it fits the storyline.