The Poseidon Adventure. And I started thinking about the character played by Gene Hackman.
If you haven't seen the movie, it's about a ocean passenger liner that is capsized by a huge wave (they may have called it a "tidal wave") and an intrepid group of passengers that try to escape, lead by a priest played by Gene Hackman. The priest is a bit of an iconoclast, arguing that you can pray all you want to God but you'd better be ready to help yourself, first. When I was 12 years old watching this movie I didn't care. I didn't want to listen to this priest argue theology. I wanted to get to the fun, excitement, adventure, and a 19-year-old Pamela Sue Martin. In other words, I didn't care about character development. And I'm sure my first writings (when I was 12) also didn't have much character development in it.
You may have a grand adventure in mind but no one will care if they don't care about the characters. Without unique, flawed, descriptive characters you don't have a story. You might have characters stuck in a capsized passenger liner, but no one will care unless they can sympathize and relate to the characters. Your characters and their interaction with the environment, circumstances, and other characters must be what your story is about. Oh, sure, they manage to escape the capsized ocean liner but you're writing a novel, not a travelogue.
Even your villains need to be fully thought out. Someone once recommended asking "What do your villain's friends like about him or her." She doesn't have friends? Then you don't have a character you have a cardboard cut-out.
You hero needs a flaw (or ten) to overcome, insecurities, and a history. Your villain needs good qualities (to overcome), flaws to exploit, insecurities, and a history.
Now, if your hero speaks to a bartender to order a whiskey I don't expect the bartender to be a fully developed character (I call those "NPCs" from my role-playing game days). But he could have a unique manner of speaking or a limp or be surly or like to tell jokes.
It's characters that drive your story. Yes, they might have a grand adventure, but no one will care about the adventure if they don't care about the characters.
*My WIP is a western/fantasy mash-up, and that reminded me of my friend, Judith Ann McDowell's latest book, a pure western. And I remembered she was talking about a character named Stella who was a nasty prostitute and I was thinking I wished she'd named her something else because I like the name "Stella," I think it's pretty, like Stella Stevens, the actress who was quite pretty in the movie The Poseidon Adventure playing a prostitute who married a cop. And that's how my mind works.