The iceberg below the surface is much more important than the iceberg above the surface. It’s the edifice below that’s going to sink your ship. It’s also that mass that keeps the smaller, visible mass above the water stable. Without it, the smaller mass up above will sink, sag, topple over, etc.
I think the iceberg idea is one of the main problems with fantasy these days. You need to do your homework first before writing your book. And by homework, I mean create the history of the story. Write a history that no one will ever read except you (or, if you become famous like Tolkien, you later publish as your own Silmarillion). You must figure out What Happened Before. If your farm boy (named Ned or Zingo or S’lart) lives in a little town way out in the middle of enchanted nowhere, how did your town come to be? Who first moved there, two hundred years ago, built himself a cabin and began farming turnips? Why did he move there? Who rules the kingdom in charge of his town and where did they come from? Who murdered whom five hundred years ago? How did evil come to dwell in the mountains to the north?
Etcetera, etcetera, etcetera.
You must write this kind of history. It will be the hidden part of the iceberg that will keep the visible top stable enough for visitors (readers, not penguins or polar bears) to tromp around without getting tilted off into the drink. If you possess a history before you start writing your magnum opus, you will be able to write in serenity; you will be able to write with proper motivations for why such-and-such a kingdom behaves in this diplomatic fashion, why this elf would respond in this way to that dwarf, why evil behaves the way it does, and on and on.
Your story motivations, whether they are hidden or not, must be consistent and stable and logical. If they are not, your story is going to be a half-baked piece of limp Baked Alaska. Seriously. Writing by the seat of your pants only gets you so far. It might be fine if you’re Jack Kerouac and you are writing some drug-addled piece of road trip asphalt. If you’re writing fantasy that involves world-building, civilizations, good and evil, you need deep foundations or you’re in trouble. Without those foundations, without the hidden iceberg, your story will ring false and feel like a juvenile scrawl.
So, before you start writing about Kloogle and his band of misfits, off on a quest to save the world, sit down and write your history. Write the whole blasted thing and then stick it in a drawer, never to see the light of day, before you deal with Kloogle. I realize this approach will take you a lot longer. You won’t be able to get your manuscript uploaded onto Amazon or shot off to an agent (good luck with that) in several months time. It might take twice as long. It might take a year more. So what? If you’re going to write a book, do it well, for crying out loud.
Build your hidden iceberg first. If you don’t, you’ll be standing on your visible iceberg, proudly surveying the ocean, when it’ll suddenly flip over (due to not having that natural keel) and the killer whales will feast on you.