Many of today's readers say they read for character, which explains the reading public's great willingness to buy series books. The conventional wisdom is this: Multiple books featuring the same protagonist (and supporting cast) allow the characters to develop and grow, not only within each novel but also from book to book.
This is utter nonsense.
In genre fiction, it is rare indeed for characters to develop substantially within a book, or even over the length of a series. Think about it: Most mysteries take place within a relatively short time period--a few days, a week or two, maybe a month. How many cases investigated by our fearless sleuths really have such personal or social relevance that the results cause a sea change in their personality by the epilogue? How many of our series characters have life-changing epiphanies in every book?
In the classic days, our sleuths didn't have much of a life, because they weren't expected to. The emphasis was on the story and the puzzle, on the sleuth matching wits with someone exceedingly clever (but ultimately not clever enough to outwit our sleuth). Thus Hercule Poirot and Jane Marple remained delightfully unencumbered; we rarely heard anything about Campion's personal life; and so forth.
Now, in the age of psychology, readers are looking for someone they can relate to or like. That's really the key thing, not the false idea that said character "develops." Reviewers and readers, I think, often use the phrase "character development" when they mean "character likability" or "character similarity to myself, or the way I think and behave, or my own self-aspiration." Thus the same folks looking for "complexity" in their characters will turn against them if they do something unlikable, or unthinking, or naive. Readers today, more than ever, have an expectation of what they want their characters to say, do, and think; and this situation makes for great complications in the life of the poor novelist. It's almost as if readers want to direct writers in terms of how their characters behave and what they do, rather than sitting back and being taken on a journey of the author's creation.
I should not be overly general here. Certainly most readers respond to character complexity, to those little inconsistencies of thought and action that make a character more than a cardboard cutout. But complexity, or depth, I would argue, is different than character development. Let's think for a moment about the greats. Is Adam Dalgliesh now any different than he was in Cover Her Face? Wexford's older, and he's raised two girls, one of whom is quite obnoxious as an adult, but has his fundamental character changed over 20+ years? Isn't V.I. Warshawski the same tough broad she's always been? Eve Dallas is still tough and highly competent, though she's let her defenses down a bit with Roarke. But then again she was always tough, and always had a sensitive side. Isn't Barbara Havers still an undiagnosed case of Asperger's, and haven't Lynley and his wife retained their essential snobbery over the years?
Let's look at the less-than-greats, too. Isn't Stephanie Plum the same clueless airhead she's always been? Isn't the supporting cast still a wacky bunch of stereotypes, many of whom remind us of types we have known in our own lives? Kinsey Millhone remains likable and competent. 21 books later, she has softened a bit, but we haven't really seen any change in her, have we?
No, genre fiction is not about character development. It’s about a story and a hero/heroine whom we find appealing. Today’s authors know this, I think, which is why they try so hard to create likable people who can be all things to all (or many) people. My opinion is that this M.O comes at the cost of complexity and the type of depth one typically sees in more literary novels. However, I won’t say that such characters are necessarily unrealistic. Think about the people in your life. How many of them have really changed or developed in some essential way during the course of your acquaintanceship? Maybe some, but probably not many. In my own life, I see friends and family members doing the same stupid things and behaving in the same dysfunctional ways in their 50s that they did when they were in their 20s (and I don’t exempt myself from this category).