Not interchangeable. The biggest lie that your primary school teacher told you was the one about opposites. I daresay I'll rant about that another day. The biggest lie that your highschool English teacher told you was the one about synonyms. The lie in question is this: synonyms exist. They don't. Even the sound of words, the length, the spelling, has different effects on different readers. Synonyms are one of those things that grammarians invent to try to apply rules and boundaries to language. But language, like life, is messy. It doesn't conform to rules, no matter how carefully they are defined. Know this and know it well: there are no synonyms.
Example (how proud Anne Soper would be): Pair and Couple.
To have a pair, or to have a couple, you must have two things. But not any old mismatched twain. (Twain is an excellent word. It means two things. They can be (in the immortal words of Edmund Blackadder) about as similar as two completely dissimilar things in a pod.) You can't use pair and couple as synonyms, any more than you can use anything else. (What is a thesaurus for? See below).
A pair is two matching items. A couple is two complementary items. A pair is identical. Or similar. Or associated because of some feature they share. A couple can be opposites (ahem) provided they fit together. A couple is yin and yang (if you like that sort of thing). Jay and Silent Bob are a pair and a couple.
A Thesaurus is not there to help you find a different word. If you are looking for an alternative to the word you already used then you used it wrong the first time. A Thesaurus helps you to find the right word. The aposite word, by looking up a word that you know isn't quite right.