I've been re-reading The Cuckoo's Calling by Robert Galbraith. One of the things that has struck me on this re-read is the way that JK Rowling creates well-rounded, three dimensional characters.
We've all read about the importance of having both external and internal conflict in a novel. One of the things that works about The Cuckoo's Calling is the fact that Rowling has crafted inner lives for her protagonists that feel real and help connect the reader with the story.
As a reader, it's rarely the mystery that stays with me after reading a crime novel. Years later, when the plot and red herrings have become dim memories, it's the characters that linger. Whether it's the dour Kurt Wallander, the driven Lucas Davenport or a private detective like Cormoran Strike, it's the characters that resonate.
In The Cuckoo's Calling, it's the bulky figure of Cormoran Strike hobbling through the streets of London and his tenacious partner Robin.
From a writing perspective, it's interesting to consider how Rowling builds identification with these characters. When we're alone, our thoughts return inexorably to those inner conflicts and problems that preoccupy our mind. For Cormoran Strike, it's the end of his relationship with Charlotte; for Robin it's her dissatisfaction with Matthew and her desire for marriage.
While there's a lot to be said for the dense plotting of the novel and its intriguing mystery, it's these personal conflicts that make the protagonists feel particularly real and make the reader invest in their story.