No, they aren’t always interchangeable. In particular, you can’t use “very” to modify a verb, e.g., “Somehow, we very seem to over use those two words” would be wrong. In addition, “really” can be used with nouns and phrases, e.g., “The snark was really a boojum” or “The road was really one hundred miles long.”
Pedantry: Really technically means “in reality, actually” as in “He is really hurt, he’s just pretending not to let it bother him.” From this it was only a short hop to its use as an intensifier, largely synonymous with very, in the informal and colloquial registers. It should be noted, however, that it remains inappropriate as an intensifier in a formal register, e.g., in printed non-fiction, as yet. (Understand that last sentence not as prescriptive but as descriptive of a social attitude on usage).
English has had a history of using intensifiers from the, um, very earliest days of the language. “That sure is a nice car” has both an expressiveness and a positiveness lacking in “that is a nice car,” which could mean anything, including being a polite way of expressing a negative.
The main reason for this is that English lacks an intensifying case for verbs. If you could simply write “ise” in these sentences, as in “that ise a nice car,” everybody would immediately understand exactly what you meant. You can’t, so a swarm of intensifiers have emerged over the years to do the work.
Intensifiers themselves aren’t words with much beauty or rhythm. Pretty ugly is an intensifier and sounds odd. It is most definitely not an oxymoron, no matter how many illiterates say it is, but it’s not good writing. That’s why Twain advised writers to cut “very” and why Hemingway said to strike out all adjectives. Good writing gives you that sense of positive expressiveness without the lazy route of letting the intensifier do the work.
But you aren’t Twain or Hemingway. Unless you are trying for good writing in the literary sense, there is no need to worry about intensifiers. They are fine. They are perfectly fine. They are totally fine. They are very fine. They are damn fine. Why should anyone spend time polishing and rewriting prose to get the ideal and exact combination of words for a post here or in 99% of the writing that ordinary people ordinarily do?
As usual, this is not a grammar issue. It is a style issue, Style is what you need it to be. There are no rules. Whether you add an intensifying adverb to a post is about the last thing you should be thinking of.
It can be lazy, and sound a little childish. For example, instead of saying ‘very big’ it’d be more expressive to say ‘enormous.’ OTOH, occasionally you want to choose the simplest of language, and sometimes there isn’t a well-known alternative you can use; ‘very’ and ‘really’ really do have very valid uses.
Curiously, using very or* really* may reduce the emphasis in a statement. Who would you trust more, the person who says “I am certain” or the one who says “I am really certain”? Either you are certain or not, and the qualifier slightly reduces the level of certainty.
My English teacher told me that the word “very” rarely adds value. In the majority of sentences, removing it improves the message. “I am angry” is more forceful than “I am very angry.” The same applies to “really”.
I remember that PG Wodehouse used to write a chapter of a story, then delete one third of the text. (That’s why his writing is excellent, with no wasted language. And also enormously funny.) He always targeted the word “very” when doing this deletion.
Similar when hearing less well-spoken speakers, you’ll notice the origins of “really” more specifically. “That’s real pretty.” “That was real smart.” Real, true.
Verily, verily, this is a most interesting discussion, with rather valid points made by nearl7y everyone. I am truly pleased at how informative it’s been.
The word *very *can be overused. It isn’t so much that you shouldn’t use it, but you shouldn’t use it so often that its impact is cheapened. The word *extremely *is overused even more so. Once you describe something as extremely, you have nowhere else to go.
One usage of *very *I always found interesting was “It’s not very good.” Rather than meaning, “It’s good but not *very *good,” it means, “It’s not good at all.”