What a wonderful observation. It is hard, and I'm guessing I'm still pretty bad at it. But I'm starting to see the contours of the problem, and maybe even one or two of the steps I need to take to get better. Based on my progress so far, this will be a life-long tutorial.
Here's how I see the problem. I have characters in my head, and while I recognize that they should behave like stop-action plasticine figures I move around precisely the way I want, that isn't entirely what happens. They don't defy me, but they do reveal themselves in unexpected ways while quietly rejecting some of the elements of motivation and plot I try out. It feels a bit like trying to dress a doll in the wrong-sized doll clothes.
It is possible my characters are actually boring people that I merely delude myself into thinking are interesting. It's much more likely, though, that they are interesting people that I'm failing to portray in a strong enough way.
In their efforts to write in two dimensions, I suspect some authors cheat a tiny bit. I don't have any specific books in mind with these stratagems, but see if these fit any books you've read recently:
These aren't cliches, necessarily, but they are coded cues -- read any of those labels in a back cover blurb and you immediately have a shortcut into the book. It's up to the author to take the character beyond his or her label, so that we see more than the superficial framework. Some authors do that beautifully, some just slap on the counter-label (e.g., the aristocrat spy who is clueless about women; the jaded rake whose moral code requires him to bond exclusively, etc.), and some authors merely explore the label for 200+ pages, then hey, presto! the character wakes up, reforms, and falls in line in time for the HEA.
Huge Drama -- Nothing wrong with a lively plot, of course, but sometimes authors can overdo the characters' backstory. I can see the appeal; nothing like a breathtaking past to make a character seem really interesting. But the risk is that either the backstory overloads the character with motivation and conflict -- too much childhood trauma and it's hard to believe that the relationship will work -- or the past is dramatic but unrelated to the romance.
In my current work in progress, the heroine has an identical twin sister. Twins aren't inherently dramatic, so what if I add a childhood bout of leukemia, where one twin donated bone marrow and saved the other twin's life? Now that's drama! But why? I figure my fictional twins are already bonded enough & devoted enough to serve the plot. I don't assume all identical twins are like that, but mine are. They're just bonded in a non-dramatic way. It's just that they are potentially boring; I get that.
Similarly, my hero grew up in Hollywood. What if I had him breaking the law as a juvie, and his high-powered dad had to pull strings to get my guy off? Ooh, that would surely set up some father-son conflict. It's just that's not my hero; he's not like that. He's got his own identity issues with a famous father, but he's not the type to have broken the law. (He breaks some rules in the book, but it isn't done in the spirit of "That'll show Dad.") Again: potentially boring . . . if I can't make the conflict he does have seem very real.
» ¤ «
I'm not a purist about this. I have adored books that used any or all of these conventions. I don't congratulate myself for not using one myself. I just -- well, can't. These are the doll clothes that don't fit my characters.
So it's back to work for me: trying to express in two dimensions what I can easily imagine inside my head, and in the process, conveying to the reader what makes these people interesting and their romance compelling.
It's a bit like a roller-coaster ride, I have to admit.
Speaking of which, here's the ride on the coaster pictured above: the New York New York ride in Las Vegas: