Writing is an exercise in discipline.
When you sit down with an idea for a story, you know all there is to know about your characters. Occasionally they'll surprise you and come up with something you weren't expecting, but generally you know where they've come from, where they're going and how they're going to get there. You also know where your story is going. You know (sort of) where the black moment will come in, you know where the tension will be, you know where the love scene or tenderness will come in. That's plotting, right? Well, here's where the discipline comes in.
Since you know everything there is to know about your character, you want to share that with the reader. However, the reader doesn't want to know that the reason your character doesn't eat salads is because of a food fight when they were 13 that resulted in them wearing ranch salad dressing stained shoes for a week. That's just an example, but NOT telling your reader that is definitely skimming the fat from your story.
I have a tendency to do what's called an info dump. There's just so much I want you to know about my character and their life up to this point that I tend to toss it all into the story. You, the reader, don't care about that. You want to know what's happening NOW. I've had to be very conscious of NOT doing info dumps in stories.
George Santayana said, "Those who cannot learn from history are doomed to repeat it." I'm trying to avoid that in my writing. Yes, it matters to me that the main character won't eat salad, no it doesn't matter to you the reader. You'll simply think I'm showing you a character quirk and you won't delve into the "why."
So, I'm skimming off the fat in my current writing. You'll never know the entire history of the character you're reading about unless you write to me specifically and ask me about that character...even then I may not share it all.