The first line is: "There is no client as scary as an innocent man." There is no location, no time, and no name in this sentence. Instead of describing a particular moment or person, this sentence asserts a conclusion. From some experience or observation, whoever is speaking believes something about clients, fear, and innocence.
Who has clients who might be innocent? Most likely, a lawyer does. Who is afraid when they are? Again, a lawyer. The narrator worries the most when their clients are innocent.
That's a bit of a surprise. We expect that someone innocent is safe and well-behaved; a guilty client could be murderous or violent or scheming. Why does the narrator think innocent clients are scary? That creates a mystery that draws us to read on.
The words doing most of the work here are client, scary, and innocent. Those three are the most distinct, and the last two set up the mystery. Will we see an innocent and scary client in the rest of the story?
Michael Connelly heard this line from a lawyer and knew it was his first line and also the thru-line. Connelly wrote that he had been "searching the whole time for the thru-line, the engine of the story." Once he heard this, he "started putting together a story that would prove what the lawyer had told me." Because he had been planning to write a novel about a lawyer, he was spending time with lawyers, and he was ready to catch the line when it came.
I like the image of a sentence as an engine. When that happens, the first sentence captures the essence of the story. This one promises that we will see an innocent man in a scary situation. It's a strong start to a good story.