What happens when we read about a character? There's a roundness we get, an understanding that grows the longer we know of their existence. We get to know them by their quirks.
Sure, everyone comprehends and so describes an individual differently. If you and I both meet Bob twice, we may or may not have different opinions on him. This is especially true if we've encountered him in varying settings. If we each meet him 10 times, our description of him might be a closer match. We both have different interactions, but it evens out over time.With a book's character, we might then expect an even more similar comparison. After all, each reader is exposed to the same interactions word for word. Good characters, though, will leave every reader with a different impression, just like real people. They become the most familiar of extras in our world, although their stories never interact directly with our own.
I'm not taking about protagonists, either. If your protagonist doesn't elicit a real-person vibe, get that right first. I'm calling to mind the third tier characters--the ones who disappear for hundreds of pages at a time. The ones who were convenient and eased plot friction, but whom we rarely contemplate in reflecting on our hero's success.
One reader might find such a character flat. That doesn't make him so. As long as another reader can look between events and see the character's influence of growth, that character becomes complex. The very fact he can be viewed as shallow then adds to his fullness.
How many extras do you know in your day-to-day life? And all of them real people.