I am a first time novelist with writing experience. I have an idea for my novel that begins with an important scene which actually plays out in the middle of the novel. Should I write this as a prologue? If so, would it be appropriate to start Chapter One with "Two years earlier..."?
It can be done well, but there is literary danger in beginning with "An Important Scene", as opposed to an unimportant or even forgettable scene. The danger is in the utter lack of audience engagement.
Before a novel (or movie) begins, the reader knows nothing about the characters. They are willing to learn or they wouldn't open the book, but they don't know who the good guys are, who the bad guys are, what the setting is like (fantasy, dystopic, real-life, etc).
We give them clues; we almost always open on the hero (by "open" I mean the first human or human-like being they see). That gives us something to root for. But they are not invested then, they don't know if they care for this hero, like her or dislike her.
If the author opens on a big important scene there is generally too much going on for the reader to process, and the writing tends to be poor because the author is overloading dialogue and exposition with world building and character building that inevitably interferes with and slows down the big scene.
It can fall flat. This is particularly an error made by writers that have been "living with" their characters for some time already, and already love them through knowing them and their quirks and morals and history. What seems clear and natural to the writer, then, in the way the characters are behaving in this big scene, is not at all clear to the reader. The reader is sitting in his living room and from the East comes a squad with a big "A" on their shirts, from the West comes a squad with a big "B" on their shirts, and they start brawling and killing each other. It's just a confusing mess to the reader, and boring, even though this all makes perfect sense to Team A and Team B from the author's POV.
You do have to open with something, but generally the mechanics of a novel or movie tend to open on "The Normal World" for the protagonist(s) for about 5% of the length. There can be conflict to drive the reader through this 5%, but it can be "throwaway" or forgettable conflict; just day-to-day problems for them to solve. The car won't start. The power went out and they woke up late because their alarm did not go off.
It is in that opening we can do a lot of world-building, setting, and character development; then we toss them on the hot grill. Then the reader cares.
I would suggest, instead of flash back or prologue, You open normally with a character introduction, do the world building; but the first complication (around 15% story mark) is your big important scene. Then, just do what JK Rowling did in her first book: Two Years Later. (Hers was ten years, giving Harry time to grow up).
Presumably your hero was doing something two years ago; even if they need to be tangential to the big important scene. Or there are other ways to handle it; give us a short story (10% to 15% of the novel) in which the hero dies; then drop your "Two Years Later" line and re-open on your new hero and their normal life.
It is generally a mistake, for reader psychology, to start talking about "Two Years Ago". What is in the past is done and over, whatever you write sounds like a history lesson and is a pain to read. It is similar to watching a taped sports match when you've already been told who won and by how much; it feels boring. You could have enjoyed it without knowing how it ends, even though rationally speaking the outcome was predetermined; but we are irrational beings, and knowing the outcome makes a difference.
Write it in present tense, it is happening now, and we don't know how it will turn out. Again, obviously, the book is already printed and how it turns out is not in question. But it makes a difference in reading, and the reader will not mind finishing a chapter, turning the page to find "Two Years Later". They won't skip a beat.
I feel it would be appropriate to start with the something in the vein of "two years earlier" though you may just want to date both events and let the readers work it out, that's more of a personal style thing than a one size fits all.
The issue I see is that if you put that much story between the telling and the event you're either going to have to tell the tale of that scene twice. You could do this by repeating the passage, writing it from another point of view or paraphrasing it. But if there's that much story between the telling and the actual event you won't get away with simply reference it offhand like a lot of stories do when they're telling multiple interwoven narratives close together in the book structure that were separate in time.
Don't toss out any literary device you have.
Yes, this can work, and work well. In TV it's called the "How we got here" trope. Some examples are The Emperor's New Groove, Fight Club, Inception, and Matrix Reloaded. I believe one of the Terminator movies did this as well. Memento is a famous example of learning an entire story backwards.
One advantage of this trope in literature is that (in books) it has not been sorely overdone.
The answer to whether this is a good idea for you in your current project will depend on (1) the genre you are writing in, (2) the event, (3) your execution, and (4) probably other factors.
Depending on the execution, the layout of the chapters be almost any which way you prefer. I've heard of a book that directs readers to go to chapter thirteen after reading chapter one. Then to chapter six, then four, then fifteen.
It can also work to leave the important scene in the middle--and that order of events would be more typical.
Writing out of order, and then re-ordering later, can also work.
The answer is yes, but you don't need to be wedded to the idea.