Whatever genre you write, you (probably unconsciously) write people at inflection points in their lives. It would be pretty dull, after all, to write about people living their everyday life, and then ... The End. (I have read one book a bit like this, the first in a very popular sort of "cozy women's fiction" series, that to me was like watching paint dry but clearly wasn't for others, so I guess there's an audience for well-written books of every stamp!) Inflection points do tend to make an impact on us. I could tell you with some certainty how I was changed by four or five very traumatic episodes in my life, things that sometimes took mere seconds to occur. And how I was changed in a much more positive way by some relationships, romantic or otherwise.
Those are the kinds of changes I write in romance. Not huge things where somebody is a different person. Ways in which, often over the course of months, your relationships, romantic and otherwise, open you up to a fuller expression of yourself. A better version of the "you" you always had the potential to be. It isn't that the other person changed you. It is that your experiences changed you, including the traumatic ones. Perhaps losing somebody you loved made you more aware of how precious life is. Perhaps taking responsibility for a child made you realize your capacity for love. Perhaps moving to a new country with a different outlook on life shifted your outlook as well.
Lots of things happen in our lives. Writing the more hopeful outcomes of those inflection points is one of the joys of mine. But to do that, you have to know your characters, and as Bite The Dusty says above, the changes have to act on them in an organic way.
The same thing is true in thrillers, Westerns, fantasy, mystery ... many genres. You're hopefully always writing those inflection points, and the characters are reacting to them in realistic ways, and hence sucking the reader in. I just finished a technothriller by an extremely popular author, the first I'd read. Although that is one of my favorite genres, the characters might as well have been cardboard cutouts. The plot was interesting, but being unable to care about the characters made the book less engaging for me. I suspect the author's characterizations got better over time, or he wouldn't be so popular. I may give it another try. We'll see.
