The trick to writing believable characters is in knowing their motivations.
Why they react the way they do - is because it's the way your reader will react.
That's IF you have studied motivations and human reactions before you start writing.
While you can study all manner of sources for motivation, a couple of good sources are the truly great copywriters and also the top self-help authors such as Napoleon Hill. Because Hill wrote for sales personnel as well. And both are interested in getting people motivated to buy what they are offering.
Self-help and Personal Improvement areas have many authors and speakers like Lester Levenson, who has narrowed all human reactions to four wants and an underlying fear. And his particular route is one that will lead directly to enlightenment, according to Rhonda Byrne.
So there's that. Of course, that doesn't mean that enlightened people read books. Or your particular books.
Where this human motivation study-trail gets interesting is looking back to Joseph Campbell's Hero's Journey. He recorded a series of videos in the years just before his death. (The Mythos Series.) These videos give the background data he used to develop that monomyth.
Why that's interesting is found in the same legends and myths that composed his theory and book. What he discovered were the constant and unchanging human reasons people do things. Because, just as humans haven't really changed over the last 10,000 years of common history, the various myths and legends have interesting commonalities about how people react.
Things that did change were those that religions, governments, and technology made. However, their changes were more like the old phrase - "The more things change, the more they stay the same." There are certain points where "management changes" made alterations in how the society around them operated. And improved the quality of life. But those superficial changes didn't affect how people themselves continued to react.
And this is the core reason to study legends, philosophies, and religions. Because reading "genre" fiction (those books "written to market") is only going to give you more of the same overused cliches and "tropes". Perennial-selling classics are built on immortal themes. Such classics don't quit selling when you quit advertising.
The point is to sleuth out the natural reactions to changes, rather than the continuing environmental changes around them.
Because those natural reactions haven't ever changed. And people then expect those ever-present reactions - because that's what they'd do in that situation. So that gives you a clue about how how your readers get involved in your stories.
This is the point that Christopher Vogler points out in his "Writer's Journey" (3rd Ed.) - that well-written books produce certain reactions in readers, "An effective story grabs your gut, tightens your throat, makes your heart race and your lungs pump, brings tears to your eyes or an explosion of laughter to your lips. If I wasn't getting some kind of physiological reaction from a story, I knew it was only affecting me on an intellectual level and therefore it would probably leave audiences cold." (Note: he expands on this in this edition's Appendix.)
Now, there's some worlds of difference in reactions according to etiquette in different cities, different social classes, rural vs. urban environments - even the cultures of nations and localities. But those are just the veneer - the details of the interactions which any author will research.
Those exist on top of the normal human reactions that people have. Natural reactions can't be faked. Etiquette rules can be - and it's obvious. People know that - and expect some false clues and false reactions in mysteries, in flawed heroes, and in villians.
This research and knowledge doesn't lighten the work of an author. But once you've taken the time and mastered the core motivations that produce all action, and the natural reactions those actions normally cause, then your characters will then do the "normal" thing that seems obvious to your readers. And that story will then produce that "gut" reaction that confirms the realness and accuracy of your writing.
Now you know.
Here's hoping your next book is now even more outstanding than any others you've ever written.